Fix segfault in test_planner_random_node/test_planner_costmaps_node (#…

…1640)

The problem appears while addressing zero-length path vector
after planner failed to create a plan.

nav2_system_tests/src/planning/planner_tester.hpp

@@ -83,6 +83,12 @@ class NavFnPlannerTester : public nav2_planner::PlannerServer
     }
     try {
       path = planners_["GridBased"]->createPlan(start, goal);
+      // The situation when createPlan() did not throw any exception
+      // does not guarantee that plan was created correctly.
+      // So it should be checked additionally that path is correct.
+      if (!path.poses.size()) {
+        return false;
+      }
     } catch (...) {
       return false;
     }