assertion failure in WITHIN GROUP patch

create table big(a int);
insert into big select generate_series(10000000,1,-1);
vacuum big;
vacuum big;
select percentile_cont(0.5) within group (order by a) from big;


TRAP: FailedAssertion("!(pointer != ((void*)0))", File: "mcxt.c", Line:
699)
LOG:  server process (PID 18750) was terminated by signal 6: Aborted
DETAIL:  Failed process was running: select percentile_cont(0.5) within
group (order by a) from big;

Regards,
    Jeff Davis