Remove hash_destroy calls in hash_create's failure paths. As noted by

Remove hash_destroy calls in hash_create's failure paths.  As noted by
a Coverity warning, these are risky since the hashtable isn't necessarily
fully set up yet.  They're unnecessary anyway: a deletable hashtable
should be in a memory context that will be cleared following elog(ERROR).
Per report from Martijn van Oosterhout.