Fix possible future cache reference leak in ALTER EXTENSION ADD/DROP.

Fix possible future cache reference leak in ALTER EXTENSION ADD/DROP.

recordExtObjInitPriv and removeExtObjInitPriv were sloppy about
calling ReleaseSysCache.  The cases cannot occur given current usage
in ALTER EXTENSION ADD/DROP, since we wouldn't get here for these
relkinds; but it seems wise to clean up better.

In passing, extend test logic in test_pg_dump to exercise the
dropped-column code paths here.

Since the case is unreachable at present, there seems no great
need to back-patch; hence fix HEAD only.

Kyotaro Horiguchi, with test case and comment adjustments by me

Discussion: https://postgr.es/m/20200417.151831.1153577605111650154.horikyota.ntt@gmail.com