Fix debug build. (#570)

cartographer_ros/cartographer_ros/ros_map_writing_points_processor.cc

@@ -119,6 +119,9 @@ RosMapWritingPointsProcessor::Flush() {
       return FlushResult::kFinished;
   }
   LOG(FATAL);
+  // The following unreachable return statement is needed to avoid a GCC bug
+  // described at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81508
+  return FlushResult::kFinished;
 }
 
 }  // namespace cartographer_ros