uORBManager.cpp: Make sure the sub is registered (valid) before invok…

…ing cb

There seems to be a race condition where the callback thread is signaled
but the sub is not registered when the orb_callback thread executes.

This patches such errors, but the root cause of the race condition is
not known. Regardless, this sanity check is not wrong.

platforms/common/uORB/uORBManager.cpp

@@ -492,6 +492,10 @@ uORB::Manager::callback_thread(int argc, char *argv[])
 			break;
 		}
 
+		if (!sub->registered()) {
+			continue;
+		}
+
 		sub->call();
 	}