[Minor] Do not start new symbols when task is being destroyed/cleaned

src/libserver/rspamd_symcache.c

 		return TRUE;
 	}
 
+	if (rspamd_session_blocked (task->s)) {
+		/*
+		 * We cannot add new events as session is either destroyed or
+		 * being cleaned up.
+		 */
+		return TRUE;
+	}
+
 	g_assert (!item->is_virtual);
 	g_assert (item->specific.normal.func != NULL);
 	if (CHECK_START_BIT (checkpoint, dyn_item)) {