postgres_fdw: Add Assert to estimate_path_cost_size().

When estimating the cost/size of a pre-sorted path for a given upper
relation using local stats, this function dereferences the passed-in
PgFdwPathExtraData pointer without checking that it is not NULL.  But
that is not a bug as the pointer is guaranteed to be non-NULL in that
case; to avoid confusion, add an Assert to ensure that it is not NULL
before dereferencing it.

Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/CAEudQArgiALbV1akQpeZOgim7XP05n%3DbDP1%3DTcOYLA43nRX_vA%40mail.gmail.com

contrib/postgres_fdw/postgres_fdw.c

@@ -3507,6 +3507,13 @@ estimate_path_cost_size(PlannerInfo *root,
 			{
 				Assert(foreignrel->reloptkind == RELOPT_UPPER_REL &&
 					   fpinfo->stage == UPPERREL_GROUP_AGG);
+
+				/*
+				 * We can only get here when this function is called from
+				 * add_foreign_ordered_paths() or add_foreign_final_paths();
+				 * in which cases, the passed-in fpextra should not be NULL.
+				 */
+				Assert(fpextra);
 				adjust_foreign_grouping_path_cost(root, pathkeys,
 												  retrieved_rows, width,
 												  fpextra->limit_tuples,