utils: Use a more special value to indicate a free slot
utils: Use a more special value to indicate a free slot

The value -1 is not so special as it is also used by pcscd to mark an
invalid slot.
libccid was happy with Lun = -1 for a reader. This should NOT be the
case.

Now the special value is -42.

LudovicRousseau/CCID,LudovicRousseau/CCID,LudovicRousseau/CCID