block/vvfat: Plug memory leak in check_directory_consistency()

On error path.  Introduced in commit a046433a

.  Spotted by Coverity.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vvfat.c

@@ -1866,7 +1866,7 @@ static int check_directory_consistency(BDRVVVFATState *s,
 
 	if (s->used_clusters[cluster_num] & USED_ANY) {
 	    fprintf(stderr, "cluster %d used more than once\n", (int)cluster_num);
-	    return 0;
+            goto fail;
 	}
 	s->used_clusters[cluster_num] = USED_DIRECTORY;