BUG #16281: LN() function inaccurate at 1000th fractional digit

The following bug has been logged on the website:

Bug reference:      16281
Logged by:          Justin
Email address:      anyhowstep@hotmail.com
PostgreSQL version: 12.2
Operating system:   Ubuntu 18.04.2
Description:

Given this,

```
SELECT

LN(484990182159328900690402236933516249572671683638747490717351807610531884491845416923860371219625151551889257298200816555016472471293780254009492949585031653913930735918829139712249577547959394351523545901788627247613322896296041868431769047433229466634098452564756860190085118463828382895145244362033728480588969626012192733802377468089120757046364393407262957242230928854711898925295251902007136232994524624903257456111389508582206404271734668422903183500589303866613158037169610592539145461637447957948521714058034772237111009429638870236361143304703683377693378577075353794118557951847394763531830696578809001981568860219578880229402696449243344235099860421846016326538272155937175661905904288335499593232232926636205909086901191153907183842087577811871344870731324067822883041265129394268082883745408414994.8967939438561591657171240282983703914075472645212002662497023142663831371447287624846942598424990784971781730103682951722370983277124599054059027055336437808366784501932987082321905202623642371063626378290734289114618092750984153422293450048717769065428713836637664433167768445609659527458911187829232316677137895259433038764404970599325009178297626038331436654541552998098529141205301472138026818453893127265938030066392881979113522757891639646670670272542401773230506961559808927249585675430838495658225557294666522469887436551840596777627408780618586500922973500018513068499587683746133637919751545157547095670767246977244726331271787622126889459658539988980096764323712767863722912919120929339399753431689512753214200090670880647731689804555417871258907716687575767185444541243606329768784843125926070743277339790277626515824924290352180761378846035233155198504033292692893297993698953705472933411199778880561376633444249703838589180474329586470353212010427945060694794274109764269805332803290229)
```

Pg's result,


1864.3702986939570026328504202935192533137907736189919154633800554877738455118081651650863235106905871352085850240570561347180517240105510505203972860921397909573687877993477806728098306202020229409548306695695574102950949468160529713610952021974630774784174851619325758380143625473386495586347322798415543385655090746985183329114860118551572428921774322172798724455202876781611633419444058398798142214904998877857425038669920064728855823072107227506485770367799671977282350083029452784747350395161797215115525867416898416360638482342253129160308632504217096916335590470843180746834864303790913372081974355613359678634194879425862536147988835528973291020680020540866655622823550861337486588647231688134992810403147262346312159819432914207194632564009749236609081399504118359354620598232725290537215007867979331582119891661859015726276335168158288396939655310210558566592649049602925182137256134162660116182293851038854455437841571331011002023088829768308520393956515509475418031437505751407687618234418263

The last digit should be a 2, but pg thinks it's a 3,

If pg supported more than 1000 decimal places, the sequence should continue
...823441826 2 4968...

The 1000th fractional digit is 2.
The 1001st fractional digit is 4.
It should round the 1000th fractional digit to 2.

I'm not sure how much people care about results at 1000 decimal places,
though.
I'm not affected by this minor rounding error, I only discovered this while
working on something else.

-----

Other values that produce behaviour,


87190145885430429849953615409019208993240447426362428988181639909267773304254748257120061524000254226856815085523676417146197197996896030672521334101413071112068202429835905642444187493717977611730127126387257253646790849384975208460867137315507010888782632024640844766297185244443116696943912406389670302370461137850160539373600494054874979342373255280815156048999900951842673141766630630919020492255966628630634124452614590400422133958133100159154995520080124736657520969784129924799670552560034302960877087853678350801769339861812435411200669026902417951572668727488315537985378304242438181615160041688723201917323705450185975141141262578884689500612295576288125956289035673242989906973367691922065122033180281670221390667818909912035903387888639331486823729897326624516015340.0330856710565117793999512551468220085711713631167607285185762046751452975325645379302403715842570486302993296501788672462090620871511446272026693318239212657949496275318383141403236705902077406660768573015707706831878445598837931116223956945944726162551477136715847593742032488181481888084716920605114101902724395659898621880016853548602514706686907951229872573180602614761229992106144727082722940736406782659562775289407005631298246624198606031298081220736931229256511054595028182057216042683060059115371651410352645266000330509331097811566633211452233019461903115970558624057877018778178814946285827512359903934291318219271464841957435711594154280905473802599888081783098187210283997106131616471807951265003903143099667366508222327805543948921694362089860577380749774036318574113007382111997454202845559941557812813566442364810680529092880773126707073967537693927177460459341763934709686530005721141046645111784404932103241501569571235364365556796422998363930810983452790309019295181282099408260156


93936642222690597390233191619858485419795942047468396309991947772747208870873993801669373075421461116465960407843923269693395211616591453397070258466704654943689268224479477016161636938138334729982904232438440955361656138189836032891825113139184685132178764873033678116450665758561650355252211196676137179184043639278410827092182700922151290703747496962700158844772453483316974221113826173404445159281421213715669245417896170368554410830320000019029956317336703559699859949692222685614036912057150632902650913831404804982509990655560731349634628713944739168096272097122388116038119844786988276635032016787352796502360718569977397214936366251320294621522016.6483354941025384161536675750898007896744690911429670830432784905421638721478353275821072200938900938046264210604940707974410950770029535636602548377806284157951164875821446035013896786653932045182167021839184824627082391478016195098055107001433336586881395912782883663046617432598969149948351689103230162742769845955320418573803127107923535948653168889411316007796459064267436246637115946581149511513369842911210359447262641996566147462977170742544980481275049898092152042927981394239266559286915303786701737610786594006685748456635797125029722684151298695274097006242412384086302106763844070230264910503179385988626477852818174114043927841085089058972074427820150462261941575665882880501074676800316585217150509780489224388148722603385921057007086785238310735038314861960410473809826927329368597558806004392175746233568789445929554890241140656324160187253042639339549705859147930476532359840809944163908006480881926041259363654863689570520534301207043189181147254153307163555433328278834311658232337