pgsql: Avoid possibly-theoretical OOM crash hazard in hash_create().

Avoid possibly-theoretical OOM crash hazard in hash_create().

One place in hash_create() used DynaHashAlloc() as a convenient
shorthand for MemoryContextAlloc(). That was fine when it was
written, but it stopped being fine when 9c911ec06 changed
DynaHashAlloc() to use MCXT_ALLOC_NO_OOM (mea culpa). Change
the code to call plain MemoryContextAlloc() as intended.

I think that this bug may be unreachable in practice, since we now
always create AllocSets with some space already allocated, so that
an OOM failure here for a non-shared hash table should be impossible
(with a hash table name of reasonable length anyway). And there
aren't enough shared hash tables to make a crash for one of those
probable. Nonetheless it's clearly not operating as designed, so
back-patch to v16 where 9c911ec06 came in.

Reported-by: Maksim Korotkov <m(dot)korotkov(at)postgrespro(dot)ru>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/219bdccd460510efaccf90b57e5e5ef2@postgrespro.ru
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f45bdb13397fc142bc95513783dca3e0106e57af

Modified Files
--------------
src/backend/utils/hash/dynahash.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)