Behavior of "OFFSET -1"

Before 9.3, you got an error from this:

regression=# select * from tenk1 offset -1;
ERROR:  OFFSET must not be negative

But 9.3 and up ignore the negative OFFSET.  This seems to be a thinko in
my commit 1a1832eb.  limit_needed() thinks it can discard the Limit plan
node altogether, which of course prevents nodeLimit.c from complaining:
               /* Executor would treat less-than-zero same as zero */               if (offset > 0)
returntrue;    /* OFFSET with a positive value */
 

I don't recall the reasoning behind that comment for sure, but I imagine
I examined the behavior of ExecLimit() and failed to notice that there
was an error check in recompute_limits().

This seems to me to be a clear bug: we should reinstate the former
behavior by tightening this check so it only discards OFFSET with a
constant value of exactly 0.  Anyone think differently?
        regards, tom lane