undefineskill can cause silent termination on startup

Undefining skill levels can cause the program to terminate immediately after the window opens, before 3DR logo or title screen animation shows. There's no indication of what happened in the terminal; the last line is Wrote settings.cfg.

To reproduce, undefine skill level 1 and leave the others.

Removing all skills also causes the same issue, but is probably a case of "don't do that."

After digging into this a bit, what appears to cause the issue is undefining any skill levels lower than those that are still defined. For example, undefining only 2 and 3 is alright, but undefining only 0 and 1 triggers the issue.