(flac_pictures): Fix compile warnings about description signedness.

[flac-archive] / TODO

- documentation is woefully out of date; man pages no longer built or
  installed

- at least one 'except Exception' block (in flac2mp3), doesn't
  re-raise SystemExit

        # check all print_exc and format_exc in fa-flacd.py; i think
        # for some i don't do this msg print check