Clean up some stuff in new contrib/bloom module.

Coverity complained about implicit sign-extension in the
BloomPageGetFreeSpace macro, probably because sizeOfBloomTuple isn't wide
enough for size calculations.  No overflow is really possible as long as
maxoff and sizeOfBloomTuple are small enough to represent a realistic
situation, but it seems like a good idea to declare sizeOfBloomTuple as
Size not int32.

Add missing check on BloomPageAddItem() result, again from Coverity.

Avoid core dump due to not allocating so->sign array when
scan->numberOfKeys is zero.  Also thanks to Coverity.

Use FLEXIBLE_ARRAY_MEMBER rather than declaring an array as size 1
when it isn't necessarily.

Very minor beautification of related code.

Unfortunately, none of the Coverity-detected mistakes look like they
could account for the remaining buildfarm unhappiness with this
module.  It's barely possible that the FLEXIBLE_ARRAY_MEMBER mistake
does account for that, if it's enabling bogus compiler optimizations;
but I'm not terribly optimistic.  We probably still have bugs to
find here.