mtd: nand: fix pointer NULL pointer dereferencing

In 'verify_bbt_descr()', first check the "bd" pointer, then
dereference it.

Comments amended by Artem.

Signed-off-by: Stanislav Fomichev <kernel@fomichev.me>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>