Limit Parallel Hash's bucket array to MaxAllocSize.

Limit Parallel Hash's bucket array to MaxAllocSize.

Make sure that we don't exceed MaxAllocSize when increasing the number of
buckets.  Perhaps later we'll remove that limit and use DSA_ALLOC_HUGE, but
for now just prevent further increases like the non-parallel code.  This
change avoids the error from bug report #15225.

Author: Thomas Munro
Reviewed-By: Tom Lane
Reported-by: Frits Jalvingh
Discussion: https://postgr.es/m/152802081668.26724.16985037679312485972%40wrigleys.postgresql.org