pgsql: Remove hash_destroy calls in hash_create's failure paths.

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/utils/hash:
        dynahash.c (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/hash/dynahash.c.diff?r1=1.70&r2=1.71)