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: stable@kernel.org
Signed-off-by: NRichard Weinberger <richard@nod.at>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@linux.intel.com>

drivers/mtd/ubi/cdev.c

@@ -632,6 +632,9 @@ static int verify_mkvol_req(const struct ubi_device *ubi,
 	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;