core: Testing a theory to encourage clean shutdowns.

src/ssb.connections.c

@@ -21,7 +21,7 @@ typedef struct _tf_ssb_connections_t
 
 static void _tf_ssb_connections_changed_callback(tf_ssb_t* ssb, tf_ssb_change_t change, tf_ssb_connection_t* connection, void* user_data)
 {
-	if (!connection)
+	if (!connection || tf_ssb_is_shutting_down(ssb))
 	{
 		return;
 	}