GCS_MAVLink: find next bucket to send when sending-ids emptied on msg removal

We need to find the next bucket of messages to send when we've removed
the last message id from the current set of IDs we're sending out - not
just when we've empted the bucket the messages came from in the first
place.

An internal check caught this bug - the
'next_deferred_bucket_message_to_send called on empty bucket' error

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -1151,17 +1151,27 @@ void GCS_MAVLINK::update_send()
 void GCS_MAVLINK::remove_message_from_bucket(int8_t bucket, ap_message id)
 {
     deferred_message_bucket[bucket].ap_message_ids.clear(id);
-
-    if (bucket == sending_bucket_id) {
-        bucket_message_ids_to_send.clear(id);
-    }
-
     if (deferred_message_bucket[bucket].ap_message_ids.count() == 0) {
         // bucket empty.  Free it:
         deferred_message_bucket[bucket].interval_ms = 0;
         deferred_message_bucket[bucket].last_sent_ms = 0;
-        if (sending_bucket_id == bucket) {
+    }
+
+    if (bucket == sending_bucket_id) {
+        bucket_message_ids_to_send.clear(id);
+        if (bucket_message_ids_to_send.count() == 0) {
             find_next_bucket_to_send();
+        } else {
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+            if (deferred_message_bucket[bucket].interval_ms == 0 &&
+                deferred_message_bucket[bucket].last_sent_ms == 0) {
+                // we just freed this bucket!  this would mean that
+                // somehow our messages-still-to-send was a superset
+                // of the messages in the bucket we were sending,
+                // which would be bad.
+                INTERNAL_ERROR(AP_InternalError::error_t::flow_of_control);
+            }
+#endif
         }
     }
 }