UBI: fix nameless volumes handling

Currently it's possible to create a volume without a name. E.g:
ubimkvol -n 32 -s 2MiB -t static /dev/ubi0 -N ""

After that vtbl_check() will always fail because it does not permit
empty strings.

Cc: [e-mail]
Signed-off-by: Richard Weinberger <[e-mail]>
Signed-off-by: Artem Bityutskiy <[e-mail]>

drivers/mtd/ubi/cdev.c

···
 	if (req->alignment != 1 && n)
 		goto bad;
 
+	if (!req->name[0] || !req->name_len)
+		goto bad;
+
 	if (req->name_len > UBI_VOL_NAME_MAX) {
 		err = -ENAMETOOLONG;
 		goto bad;