pgsql: Modify hash_create() to elog(ERROR) if an error occurs, rather

Log Message:
-----------
Modify hash_create() to elog(ERROR) if an error occurs, rather than
returning a NULL pointer (some callers remembered to check the return
value, but some did not -- it is safer to just bail out).

Also, cleanup pgstat.c to use elog(ERROR) rather than elog(LOG) followed
by exit().

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.35 -> r1.36)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c.diff?r1=1.35&r2=1.36)
    pgsql/src/backend/commands:
        prepare.c (r1.32 -> r1.33)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c.diff?r1=1.32&r2=1.33)
    pgsql/src/backend/executor:
        execGrouping.c (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execGrouping.c.diff?r1=1.11&r2=1.12)
        nodeIndexscan.c (r1.97 -> r1.98)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeIndexscan.c.diff?r1=1.97&r2=1.98)
    pgsql/src/backend/postmaster:
        pgstat.c (r1.81 -> r1.82)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.81&r2=1.82)
    pgsql/src/backend/storage/lmgr:
        lock.c (r1.142 -> r1.143)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c.diff?r1=1.142&r2=1.143)
    pgsql/src/backend/storage/smgr:
        md.c (r1.112 -> r1.113)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/smgr/md.c.diff?r1=1.112&r2=1.113)
    pgsql/src/backend/utils/fmgr:
        fmgr.c (r1.85 -> r1.86)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/fmgr.c.diff?r1=1.85&r2=1.86)
    pgsql/src/backend/utils/hash:
        dynahash.c (r1.55 -> r1.56)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/hash/dynahash.c.diff?r1=1.55&r2=1.56)