message: locked_queue: fix maybe derefence null warning
diff: @@ -127,7 +127,10 @@ TEST(LockedQueue, ChangeValue)
  {
  auto guard = locked_queue.guard();
  EXPECT_EQ(guard.get_front()->value, 42);
  - guard.get_front()->value = 43;
  + auto front = guard.get_front();
  + if (front) {
  + front->value = 43;
  + }
  }
  {
  