Fix vfat shortname character logic that had wrong signedness

Fix vfat shortname character logic that had wrong signedness
tests.

From Dennis Vshivkov:

  "whenever I was trying to create a file using only national
   characters for its name, it was always created with an
   uppercased shortname (provided that the name was not too
   long for 8.3, of course). Everything was fine with latin
   filenames, though."

The bug is that we check "buf[0]" as an unsigned character, but
"buf" is just "char *", which (depending on architecture and
compiler options) tends to be signed.