Program panics if any null byte is encountered.

[Uinelj]

When using prediction functions with strings containing a null byte, the library crashes due to an unwrap() on a CString creation.
However, encountering null bytes happens in the nature and while the presence of null bytes could be checked upstream, I don't think that having the program crash is intended behaviour.

This could be fixed by removing unwrap, change return types if applicable and map the error to a String.

I'd be glad to implement such a fix :)

[messense]

Thanks for reporting, go ahead fix it I'm happy to accept a PR.

[messense]

BTW, I think maybe we should use the c_fixed_string crate instead of just CString.