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>

block/vvfat.c

@@ -2865,7 +2865,7 @@ static void checkpoint(void) {
     return;
     /* avoid compiler warnings: */
     hexdump(NULL, 100);
-    remove_mapping(vvv, NULL);
+    remove_mapping(vvv, 0);
     print_mapping(NULL);
     print_direntry(NULL);
 }