GCS_MAVLink: only try to fwd packets to active channels

libraries/GCS_MAVLink/MAVLink_routing.cpp

@@ -20,6 +20,7 @@
 
 #include <AP_HAL.h>
 #include <AP_Common.h>
+#include <GCS.h>
 #include <MAVLink_routing.h>
 #include <stdio.h>
 
@@ -119,7 +120,7 @@ void MAVLink_routing::learn_route(mavlink_channel_t in_channel, const mavlink_me
 */
 void MAVLink_routing::handle_heartbeat(mavlink_channel_t in_channel, const mavlink_message_t* msg)
 {
-    uint16_t mask = (1U<<MAVLINK_COMM_NUM_BUFFERS)-1;
+    uint16_t mask = GCS_MAVLINK::active_channel_mask();
 
     // don't send on the incoming channel. This should only matter if
     // the routing table is full