Don't fcntl on an invalid fd.

[rbartlensky]

Hello! I've stumbled upon an error while using this crate

thread 'main' panicked at 'Failed to initialize inotify.: Os { code: 9, kind: Other, message: "Bad file descriptor" }'

and I figured it's probably because the code doesn't check whether inotify_init returns -1.

Let me know if I need to change anything else!

[hannobraun]

Thank you, @rbartlensky, good catch!

I think an even better solution would be to remove the fcntls completely and use inotify_init1 instead. Using inotify_init and fcntl was a workaround for a downstream issue that is no longer required (see #146 and #151 for context). However, this PR is a clear improvement, so I'll merge it.

If you (or anyone else reading this) wants to address #151 in a follow-up PR, that would be very welcome.