fix random crashes upon !reload

A struct plugin_hub_internals was falsely casted to struct
plugin_callback_data. This caused the contained commands list pointer to point to
a struct hub_info and commands->size took the value of a pointer to a struct
net_connection. Since size is increased/decreased every time an item is
added to/removed from the list, this resulted in some funny crashes.

This fix is a little dirty as it exports some internals.

src/util/list.c

@@ -32,7 +32,12 @@ struct linked_list* list_create()
 void list_destroy(struct linked_list* list)
 {
 	if (list)
+	{
+		uhub_assert(list->size == 0);
+		uhub_assert(list->first == NULL);
+		uhub_assert(list->last == NULL);
 		hub_free(list);
+	}
 }
 
 
@@ -102,10 +107,11 @@ void list_remove(struct linked_list* list, void* data_ptr)
 			hub_free(node);
 
 			list->size--;
-			break;
+			return;
 		}
 		node = node->next;
 	}
+	uhub_assert(false);
 }