[FIX] PubSub: Unregister Subscriber callback

 - Unregister Subscriber callback when removing ReaderGroup
 - Resolve segmentation fault when removing ReaderGroup

Change-Id: Ia5c7aefa706eb2cdbb990a89c57b9444c0f761bf

src/pubsub/ua_pubsub.c

@@ -271,6 +271,8 @@ UA_Server_removeReaderGroup(UA_Server *server, UA_NodeId groupIdentifier) {
         return UA_STATUSCODE_BADNOTFOUND;
     }
 
+    /* Unregister subscribe callback */
+    UA_PubSubManager_removeRepeatedPubSubCallback(server, readerGroup->subscribeCallbackId);
 #ifdef UA_ENABLE_PUBSUB_INFORMATIONMODEL
     /* To Do:RemoveGroupRepresentation(server, &readerGroup->identifier) */
 #endif