Clear outgoingMessages on last drained subscriber

src/TopicTree.h

@@ -252,6 +252,11 @@ struct TopicTree {
             /* This one always resets needsDrainage before it calls any cb's.
              * Otherwise we would stackoverflow when sending after publish but before drain. */
             drainImpl(s);
+
+            /* If we drained last subscriber, also clear outgoingMessages */
+            if (!drainableSubscribers) {
+                outgoingMessages.clear();
+            }
         }
     }