set_deparse_plan: Reuse variable to appease Coverity

Coverity complains that dpns->outer_plan is deferenced (to obtain
->targetlist) when possibly NULL.  We can avoid this by using
dpns->outer_tlist instead, which was already obtained a few lines up.

The fact that we end up with
  dpns->inner_tlist = dpns->outer_tlist
is a bit suspicious-looking and maybe worthy of more investigation, but
I'll leave that for another day.

Reviewed-by: Michaël Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/202204191345.qerjy3kxi3eb@alvherre.pgsql

src/backend/utils/adt/ruleutils.c

@@ -4996,7 +4996,7 @@ set_deparse_plan(deparse_namespace *dpns, Plan *plan)
 	if (IsA(plan, ModifyTable))
 	{
 		if (((ModifyTable *) plan)->operation == CMD_MERGE)
-			dpns->inner_tlist = dpns->outer_plan->targetlist;
+			dpns->inner_tlist = dpns->outer_tlist;
 		else
 			dpns->inner_tlist = ((ModifyTable *) plan)->exclRelTlist;
 	}