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

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