Listing 19-6: use ptr.add instead of ptr.offset

[SuperSamus]

In Chapter 19.1 (Unsafe Rust), the line from listing 19-6 slice::from_raw_parts_mut(ptr.offset(mid as isize), len - mid)) should use ptr.add instead (beacause it asks for an usize, avoiding the conversion), just like the official implementation does.
Also, Clippy complains:

use of offset with a usize casted to an isize

note: #[warn(clippy::ptr_offset_with_cast)] on by default
help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_offset_with_cast
help: try: ptr.add(mid)clippy(clippy::ptr_offset_with_cast)