glib/bytes: relax start offset constraint for from_bytes

[BiagioFesta]

Maybe I've missed something, but it was faster opening a one-line PR for seeking feedback.

In rust this is totally valid:

let bytes = b"1234";
let sub = &bytes[4..];

But this is not in glib at the moment:

let bytes = glib::Bytes::from_static(b"1234");
let sub = glib::Bytes::from_bytes(&bytes, 4..);  // This crashes without this patch

This patch does not contradict Bytes.new_from_bytes 'cause end + 0 <= len holds.

[sdroege]

Thanks, this makes sense