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

Comments amended by Artem.
Signed-off-by: NStanislav Fomichev <kernel@fomichev.me>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>