dm-verity: fix a possible NULL pointer dereference

Fix a possible NULL pointer dereference dm_verity_loadpin_is_bdev_trusted
if the device has no table.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Fixes: b6c1c5745ccc ("dm: Add verity helpers for LoadPin")
Cc: stable@vger.kernel.org