Rover: Handle NAV_CONTROLLER_OUTPUT.wp_dist overflowing

APMrover2/GCS_Mavlink.cpp

@@ -144,7 +144,7 @@ void Rover::send_nav_controller_output(mavlink_channel_t chan)
         ahrs.groundspeed() * ins.get_gyro().z,  // use nav_pitch to hold actual Y accel
         nav_controller->nav_bearing_cd() * 0.01f,
         nav_controller->target_bearing_cd() * 0.01f,
-        wp_distance,
+        MIN(wp_distance, UINT16_MAX),
         0,
         groundspeed_error,
         nav_controller->crosstrack_error());