fix crash when called on a non-existing file

[cooljeanius]

I forget what the actual crash was that originally made me add this to the TODO, but apparently it happened?

[cooljeanius]

@kencu appears to have rediscovered this in macports/macports-ports#21227 perhaps? I think I might just need to check to ensure that argv is nonnull before using it?

[cooljeanius]

OK this ought to be fixed by 4ede21a; @kencu can you confirm?

[kencu]

yes, that works nicely

[cooljeanius]

yes, that works nicely

OK thanks, I'll bump the version and then push a new release.