s390/cio: avoid calling strlen on null pointer

Fix the following kasan finding:
BUG: KASAN: global-out-of-bounds in ccwgroup_create_dev+0x850/0x1140
Read of size 1 at addr 0000000000000000 by task systemd-udevd.r/561

CPU: 30 PID: 561 Comm: systemd-udevd.r Tainted: G    B
Hardware name: IBM 3906 M04 704 (LPAR)
Call Trace:
([<0000000231b3db7e>] show_stack+0x14e/0x1a8)
 [<0000000233826410>] dump_stack+0x1d0/0x218
 [<000000023216fac4>] print_address_description+0x64/0x380
 [<000000023216f5a8>] __kasan_report+0x138/0x168
 [<00000002331b8378>] ccwgroup_create_dev+0x850/0x1140
 [<00000002332b618a>] group_store+0x3a/0x50
 [<00000002323ac706>] kernfs_fop_write+0x246/0x3b8
 [<00000002321d409a>] vfs_write+0x132/0x450
 [<00000002321d47da>] ksys_write+0x122/0x208
 [<0000000233877102>] system_call+0x2a6/0x2c8

Triggered by:
openat(AT_FDCWD, "/sys/bus/ccwgroup/drivers/qeth/group",
		O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, 0666) = 16
write(16, "0.0.bd00,0.0.bd01,0.0.bd02", 26) = 26

The problem is that __get_next_id in ccwgroup_create_dev might set "buf"
buffer pointer to NULL and explicit check for that is required.

Cc: stable@vger.kernel.org
Reviewed-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/cio/ccwgroup.c

@@ -372,7 +372,7 @@ int ccwgroup_create_dev(struct device *parent, struct ccwgroup_driver *gdrv,
 		goto error;
 	}
 	/* Check for trailing stuff. */
-	if (i == num_devices && strlen(buf) > 0) {
+	if (i == num_devices && buf && strlen(buf) > 0) {
 		rc = -EINVAL;
 		goto error;
 	}