EKF: Don't fallback when optical flow is lost if external vision data is being used

EKF/control.cpp

@@ -336,7 +336,7 @@ void Ekf::controlOpticalFlowFusion()
 		}
 
 		// handle the case when we are relying on optical flow fusion and lose it
-		if (_control_status.flags.opt_flow && !_control_status.flags.gps) {
+		if (_control_status.flags.opt_flow && !_control_status.flags.gps && !_control_status.flags.ev_pos) {
 			// We are relying on flow aiding to constrain attitude drift so after 5s without aiding we need to do something
 			if ((_time_last_imu - _time_last_of_fuse > 5e6)) {
 				// Switch to the non-aiding mode, zero the velocity states