Fix bug in the new ResourceOwner implementation.

Fix bug in the new ResourceOwner implementation.

When the hash table is in use, ResoureOwnerSort() moves any elements
from the small fixed-size array to the hash table, and sorts it. When
the hash table is not in use, it sorts the elements in the small
fixed-size array directly. However, ResourceOwnerSort() and
ResourceOwnerReleaseAll() had different idea on when the hash table is
in use: ResourceOwnerSort() checked owner->nhash != 0, and
ResourceOwnerReleaseAll() checked owner->hash != NULL. If the hash
table was allocated but was currently empty, you hit an assertion
failure.

Reported-by: Alexander Lakhin 
Discussion: https://www.postgresql.org/message-id/be58d565-9e95-d417-4e47-f6bd408dea4b@gmail.com