fix: update engine pointers when engine crashes

[MinetaS]

Fixes #908

[Disservin]

Thanks, I wonder if we could have a unit test for this case and your other PR. I'm currently traveling so can't write one right now but will try to when I'm back unless someone already opens a pr in the meantime

[MinetaS]

Hm I don't know much about the unit test of fastchess at the moment. But testing something like my other PR is probably not suitable, because it needs

1. to check internal properties (i.e. fds) of engine processes, which requires an external observer, and

2. live engine processes, because it deals with actual Process objects.

I believe unit tests are not supposed to test such things. Simulating engine stall/disconnections might be feasible, but again I'm not familiar with this thing now...

[Disservin]

Yeah I can do it, there are already tests not really unit though, which use a random mover engine or a simple dummy one.

Regarding file descriptors valgrind has an option to track them which I think is already done in one test, I think the same logic can be reused there