After incrementing an unsigned char value, don't cast it back to

unsigned char if you want to preserve its ordering! Fixes a bug
whereby an 0xFF character in a filename could trigger an assertion
failure (since du.c would sort the directory contents correctly but
trie.c would then expect it to be sorted wrong).

git-svn-id: svn://svn.tartarus.org/sgt/agedu@8595 cda61777-01e9-0310-a592-d414129be87e