postgres_fdw: Fix test for cached costs in estimate_path_cost_size().

estimate_path_cost_size() failed to re-use cached costs when the cached
startup/total cost was 0, so it calculated the costs redundantly.

This is an oversight in commit aa09cd24; but apply the patch to HEAD
only because there are no reports of actual trouble from that.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/5C4AF3F3.4060409%40lab.ntt.co.jp

contrib/postgres_fdw/postgres_fdw.c

@@ -2625,7 +2625,7 @@ estimate_path_cost_size(PlannerInfo *root,
 		 * bare scan each time. Instead, use the costs if we have cached them
 		 * already.
 		 */
-		if (fpinfo->rel_startup_cost > 0 && fpinfo->rel_total_cost > 0)
+		if (fpinfo->rel_startup_cost >= 0 && fpinfo->rel_total_cost >= 0)
 		{
 			startup_cost = fpinfo->rel_startup_cost;
 			run_cost = fpinfo->rel_total_cost - fpinfo->rel_startup_cost;