pgsql: postgres_fdw: Promote an Assert() to elog().

postgres_fdw: Promote an Assert() to elog().

Andreas Seltenreich reports that it is possible for a PlaceHolderVar
to creep into this tlist, and I fear that even after that's fixed we
might have other, similar bugs in this area either now or in the
future. There's a lot of action-at-a-distance here, because the
validity of this assertion depends on core planner behavior; so, let's
use elog() to make sure we catch this even in non-assert builds,
rather than just crashing.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/332fdbef20b5b5f2588447793dbcc3bb9b88eb51

Modified Files
--------------
contrib/postgres_fdw/deparse.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)