GCS_MAVLink: avoid copying data twice in ftp transfers

libraries/GCS_MAVLink/GCS_FTP.cpp

@@ -123,7 +123,7 @@ void GCS_MAVLINK::send_ftp_replies(void)
                     reply.chan,
                     0, reply.sysid, reply.compid,
                     payload);
-                ftp.replies->pop(reply);
+                ftp.replies->pop();
                 ftp.last_send_ms = AP_HAL::millis();
         } else {
             return;