vvfat: Fix compilation with DEBUG defined

gcc does not like passing a NULL where an int value is expected:

block/vvfat.c: In function ‘checkpoint’:
block/vvfat.c:2868: error: passing argument 2 of ‘remove_mapping’ makes
  integer from pointer without a cast

Signed-off-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
