dm flakey: fix a crash with invalid table line

This command will crash with NULL pointer dereference:
 dmsetup create flakey --table \
  "0 `blockdev --getsize /dev/ram0` flakey /dev/ram0 0 0 1 2 corrupt_bio_byte 512"

Fix the crash by checking if arg_name is non-NULL before comparing it.

Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-flakey.c

@@ -125,9 +125,9 @@ static int parse_features(struct dm_arg_set *as, struct flakey_c *fc,
 			 * Direction r or w?
 			 */
 			arg_name = dm_shift_arg(as);
-			if (!strcasecmp(arg_name, "w"))
+			if (arg_name && !strcasecmp(arg_name, "w"))
 				fc->corrupt_bio_rw = WRITE;
-			else if (!strcasecmp(arg_name, "r"))
+			else if (arg_name && !strcasecmp(arg_name, "r"))
 				fc->corrupt_bio_rw = READ;
 			else {
 				ti->error = "Invalid corrupt bio direction (r or w)";