Don't crash upon reaching EndFile, or if observing properties.

[yaymukund]

If you reach the end of the file-- e.g. by playing past the end-- the following assertion fails triggers a panic:

https://github.com/ParadoxSpiral/mpv-rs/blob/99853ac04dd8cb890cce0b98a1094c2b873600e0/src/wrapper/events/simple.rs#L198

I changed it to return None.

Then it panics if it tries to observe a property change on an already-ended file because then the format is blank and the data is invalid. See:

https://github.com/ParadoxSpiral/mpv-rs/blob/99853ac04dd8cb890cce0b98a1094c2b873600e0/src/wrapper/events/simple.rs#L41
https://github.com/ParadoxSpiral/mpv-rs/blob/99853ac04dd8cb890cce0b98a1094c2b873600e0/mpv-sys/include/client.h#L1117-L1122

I patched that too, and now it no longer crashes when reaching the end of a file.

[ParadoxSpiral]

Thanks!

[yaymukund]

Thank you for merging this. It's all working smoothly for me :)