Panic in `get_int` if nbytes is zero

[somtochiama]

get_int panics if nbytes is zero due to attempt to right shift by 64 when doing the sign extension.

    let mut buf = &b"hello world"[..];
    let ret1 = buf.get_int(0);

Not sure if this is the intended behaviour that isn't documented. Doing the same with get_uint returns zero which might be preferrable.