If an attached bb device was removed before the actual mtd device
the code would try due to a missing pointer reset to access
the no more present bb device handle which leads to a page fault.

This bug was made visible by commit "7649473 mtd: nand: remove
automatically created bb devices" which relys on a correct
mtd->cdev_bb handling.

Signed-off-by: Heinrich Toews <heinrich.toews@wago.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>