postgres_fdw: Fix the fix for crash when pushing down multiple joins.

Commit 3151f16e was intended to be
a commit of a patch from Ashutosh Bapat, but instead I mistakenly
committed an earlier version from Michael Paquier (because both
patches were submitted with the same filename, and I confused them).
Michael's patch fixes the crash but doesn't actually implement the
correct test.

Repair the incorrect logic, and also expand the comments considerably
so that this is all more clear.

Ashutosh Bapat and Robert Haas