LGTM.com - false positive for cpp/toctou-race-condition

**Description of the false positive**

Even though the code first does a `stat` and then after that an `open` on the same file, it still afterwards checks that the `open` call succeeded before operating on the file descriptor. Thus, there is no toctou race condition. I understand that this can be considered an edge case, but still reporting it here for consideration.

**URL to the alert on the project page on LGTM.com**

https://lgtm.com/projects/g/DOMjudge/domjudge/snapshot/66789bbc091123b7a00dc368032b9dbb5f3e0e96/files/judge/evict.c?sort=name&dir=ASC&mode=heatmap#x5362a542144696a7:1