Fix cache reference leak in contrib/sepgsql.

Fix cache reference leak in contrib/sepgsql.

fixup_whole_row_references() did the wrong thing with a dropped column,
resulting in a commit-time warning about a cache reference leak.

I (tgl) added a test case exercising this, but back-patched the test
only as far as v10; the patch didn't apply cleanly to 9.6 and it
didn't seem worth the trouble to adapt it.  The bug is pretty old
though, so apply the code change all the way back.

Michael Luo, with cosmetic improvements by me

Discussion: https://postgr.es/m/BYAPR08MB5606D1453D7F50E2AF4D2FD29AD80@BYAPR08MB5606.namprd08.prod.outlook.com