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