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.

contrib/postgres_fdw/deparse.c

@@ -1112,8 +1112,10 @@ deparseExplicitTargetList(List *tlist, List **retrieved_attrs,
 		/* Extract expression if TargetEntry node */
 		Assert(IsA(tle, TargetEntry));
 		var = (Var *) tle->expr;
+
 		/* We expect only Var nodes here */
-		Assert(IsA(var, Var));
+		if (!IsA(var, Var))
+			elog(ERROR, "non-Var not expected in target list");
 
 		if (i > 0)
 			appendStringInfoString(buf, ", ");