9.4rc bug in percentile_cont

Just got a report on IRC of a bug in the array version of
percentile_cont; if two of the requested percentiles were between the
same pair of input rows, the result could be wrong or an error would
be generated.

e.g.

select percentile_cont(array[0.4,0.6]) within group (order by gs)
  from generate_series(1,2) gs;
ERROR: missing row in percentile_cont

Proposed patch (against current master) attached.

--
Andrew (irc:RhodiumToad)