Remove endless counter incrementing.

In some cases it is not enough to perform compare and exchange
and we still can get race condition in atomic incrementing.
Therefore, it is better to loose a history event than to
log error message each time.

src/roll_history.c

@@ -101,7 +101,8 @@ rspamd_roll_history_update (struct roll_history *history, struct worker_task *ta
 		row->completed = FALSE;
 	}
 	else {
-		msg_err ("internal error with history roll occured, row number is invalid: %d", row_num);
+		/* Race condition */
+		history->cur_row = 0;
 		return;
 	}