Fix clippy warning.

[jdm]

 Warning: warning: use of `offset` with a `usize` casted to an `isize`
  --> tendril/src/util.rs:27:13
   |
27 |     *dest = dest.offset(src.len() as isize)
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#ptr_offset_with_cast
   = note: `#[warn(clippy::ptr_offset_with_cast)]` on by default
help: use `add` instead
   |
27 -     *dest = dest.offset(src.len() as isize)
27 +     *dest = dest.add(src.len())
   |