Soundness issue in `iced-x86` versions <= 1.10.3

Versions of iced-x86 <= 1.10.3 invoke undefined behavior which may cause soundness
issues in crates using the iced_x86::Decoder struct. The Decoder::new() function
made a call to slice.get_unchecked(slice.length()) to get the end position of
the input buffer. The flaw was fixed with safe logic that does not invoke undefined
behavior.

More details can be found at <https://github.com/icedland/iced/issues/168&gt;.