keyboard-interactive authentication panics - with a potential fix

[ayourtch]

The handler for keyboard-interactive uses from_raw_parts(), and several fields get null pointer and zero length passed, which results in panics.

I have fixed it in my own fork here - master...ayourtch:ssh2-rs:fix-interactive-auth - is it something that you would entertain having me open a PR for ? Anything that I should update ?

(I was thinking running cargo fmt, but from experience, it sometimes behaves slightly differently depending on the version, so I deliberately avoided it).

Thanks!

[yodaldevoid]

Fixed by #333