mtd: ubi: avoid NULL pointer dereference in ubi_get_device

If the ubidetach command is given a number that passes the range check
in ubi_detach_mtd_dev, but at which index the ubi_devices array contains
a NULL pointer, ubi_get_device tries to dereference this NULL pointer
to increment the ubi device reference count.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>