microbit: BUGFIX DoS attacks on the MessageBus

- Enures an event listener is not deleted whilst a fiber is activiely processing a queue
 - Added support for resurrection of event listeners in cases where identical listeners are removed/added repetitively.

source/MicroBitMessageBus.cpp

@@ -190,7 +190,7 @@ int MicroBitMessageBus::deleteMarkedListeners()
     // Walk this list of event handlers. Delete any that match the given listener.
     while (l != NULL)
     {
-        if (l->flags & MESSAGE_BUS_LISTENER_DELETING)
+        if (l->flags & MESSAGE_BUS_LISTENER_DELETING && !l->flags & MESSAGE_BUS_LISTENER_BUSY)
         {
             if (p == NULL)
                 listeners = l->next;
@@ -455,8 +455,16 @@ int MicroBitMessageBus::add(MicroBitListener *newListener)
     {
         methodCallback = (newListener->flags & MESSAGE_BUS_LISTENER_METHOD) && (l->flags & MESSAGE_BUS_LISTENER_METHOD);
 
-        if (l->id == newListener->id && l->value == newListener->value && (methodCallback ? *l->cb_method == *newListener->cb_method : l->cb == newListener->cb) && !(l->flags & MESSAGE_BUS_LISTENER_DELETING))
+        if (l->id == newListener->id && l->value == newListener->value && (methodCallback ? *l->cb_method == *newListener->cb_method : l->cb == newListener->cb))
+        {
+            // We have a perfect match for this event listener already registered. 
+            // If it's marked for deletion, we simply resurrect the listener, and we're done.
+            // Either way, we return an error code, as the *new* listener should be released...
+            if(l->flags & MESSAGE_BUS_LISTENER_DELETING)
+                l->flags &= ~MESSAGE_BUS_LISTENER_DELETING;
+
             return 0;
+        }
 
         l = l->next;
     }