Sub: command-long DO_SET_HOME check first param is zero

ArduSub/GCS_Mavlink.cpp

@@ -1090,6 +1090,10 @@ void GCS_MAVLINK_Sub::handleMessage(mavlink_message_t* msg)
                     result = MAV_RESULT_ACCEPTED;
                 }
             } else {
+                // ensure param1 is zero
+                if (!is_zero(packet.param1)) {
+                    break;
+                }
                 // sanity check location
                 if (!check_latlng(packet.param5, packet.param6)) {
                     break;