block/vvfat: Fix compiler warning in debug code

Fix this compiler warning:
./block/vvfat.c:2285: error: comparison of unsigned expression >= 0 is always true

Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

block/vvfat.c

@@ -2282,7 +2282,6 @@ static void check1(BDRVVVFATState* s)
 	    fprintf(stderr, "deleted\n");
 	    continue;
 	}
-	assert(mapping->dir_index >= 0);
 	assert(mapping->dir_index < s->directory.next);
 	direntry_t* direntry = array_get(&(s->directory), mapping->dir_index);
 	assert(mapping->begin == begin_of_direntry(direntry) || mapping->first_mapping_index >= 0);