hash_search and out of memory

If OOM happens during expand_table() in hash_search_with_hash_value()
for RelationCacheInsert, the hash table entry is allocated and stored
in the hash table, but idhentry->reldesc remains NULL.  Since OOM
causes AbortTransaction(), in AtEOXact_RelationCache() this NULL
pointer is referenced and we hit SIGSEGV.

The fix would be either catch OOM error with PG_TRY() and undo the
hash entry, or do the expansion first and put the entry later.  The
latter is a bit ugly because we have to re-calculate hash bucket after
we decided to expand, so the former looks better.  Do you think of
other solutions?

Thanks,
-- 
Hitoshi Harada