Bump afl from 0.13.3 to 0.13.4 in /fuzz/afl.rs

Bumps [afl](https://github.com/rust-fuzz/afl.rs) from 0.13.3 to 0.13.4.
- [Changelog](https://github.com/rust-fuzz/afl.rs/blob/master/CHANGES.md)
- [Commits](https://github.com/rust-fuzz/afl.rs/commits)

---
updated-dependencies:
- dependency-name: afl
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>