ieee1275: NULL pointer dereference in grub_ieee1275_encode_devname()

ieee1275: NULL pointer dereference in grub_ieee1275_encode_devname()

Function grub_strndup() may return NULL, this is called from
function grub_ieee1275_get_devname() which is then called from
function grub_ieee1275_encode_devname() to set device. The device
variable could then be used with a NULL pointer.

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Colin Watson <cjwatson@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>