Fix infinite loop on corrupted fine-grained cache

[msullivan]

This hit me when mypyc was causing crashes after having written out
part of the cache, but subsequent runs would instead infinite loop.

[ilevkivskyi]

Good catch!

It would be great to add a test case for this, but it is probably non-trivial. Maybe add a comment in #5002, so we not forget about this?

[gvanrossum]

I think I may have hit this yesterday. I did something with dmypy like the following:

1. dmypy check mypy
2. dmypy check ../mypyc/mypyc

The daemon was started with -v logging to a separate window. It never completed (until I hit ^C).

[gvanrossum]

Should probably be merged before I cut the release branch (#5741).

[msullivan]

Turns out that we already had a test case for the case where the cache file was wrong, but we only infinite loop when it is missing.