pgsql: Fix possible future cache reference leak in ALTER EXTENSION ADD/

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3125a5baec1cf6d3aaeb8964bc3b3c49835e0452

Modified Files
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src/backend/catalog/aclchk.c | 47 +++++++++++++---------
src/test/modules/test_pg_dump/README | 2 +
.../modules/test_pg_dump/expected/test_pg_dump.out | 3 +-
src/test/modules/test_pg_dump/sql/test_pg_dump.sql | 3 +-
4 files changed, 35 insertions(+), 20 deletions(-)