commit 10640d34 upstream.

If you pass an invalid string here then you probably deserve the memory
corruption, but it annoys static analysis tools so lets fix it.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>