Fix race-condition when attempting to fetch trimmed submaps. (#812)

A simple solution for a slightly more complex scenario:
- a pure localization trajectory `X` gets finished & trimmed in the main node
- at the same time, the occupancy_grid_node handles an outdated SubmapList
  message in which a submap ID `id` of trajectory `X` is still present
- the call to FetchSubmapTextures(`id`, ...) leads to a crash

With this fix, the trimmed submap IDs are just ignored until the next
iteration (in which the occupancy grid node removes the trimmed IDs).

cartographer_ros/cartographer_ros/submap.cc

@@ -20,6 +20,7 @@
 #include "cartographer/common/port.h"
 #include "cartographer/transform/transform.h"
 #include "cartographer_ros/msg_conversion.h"
+#include "cartographer_ros_msgs/StatusCode.h"
 #include "cartographer_ros_msgs/SubmapQuery.h"
 
 namespace cartographer_ros {
@@ -30,7 +31,8 @@ std::unique_ptr<::cartographer::io::SubmapTextures> FetchSubmapTextures(
   ::cartographer_ros_msgs::SubmapQuery srv;
   srv.request.trajectory_id = submap_id.trajectory_id;
   srv.request.submap_index = submap_id.submap_index;
-  if (!client->call(srv)) {
+  if (!client->call(srv) ||
+      srv.response.status.code != ::cartographer_ros_msgs::StatusCode::OK) {
     return nullptr;
   }
   CHECK(!srv.response.textures.empty());