ref: d71ac73d8a4397c35b21ec08388a1c6f94691b64
parent: d022a1c11c185027fad937cbe9cd2c318025ee04
author: Simon Tatham <anakin@pobox.com>
date: Tue Mar 17 14:12:33 EDT 2020
Mines: add validation for negative mine count. If this gets through validation, it causes an infinite loop after gameplay begins.