mqtt: look for a reason code in all messages

instead of stopping on the first message if it does not
have a reason code, like conn and conn_ack

Was fixed in master by big refactor 0a1062f

rust/src/mqtt/detect.rs

@@ -365,7 +365,7 @@ pub unsafe extern "C" fn rs_mqtt_tx_get_reason_code(tx: &MQTTTransaction, result
                     return 1;
                 }
             }
-            _ => return 0,
+            _ => {},
         }
     }
     return 0;