pw_hex_dump: Fix bug with unaligned trailing bytes

When bytes at the end of a dump were not aligned with the "group_every"
option, half of the last byte would be truncated.

No-Docs-Update-Reason: Bugfix
Change-Id: Ibe349c9f34f41281122583a6e7dff84d4f991333
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49305
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>