vboot2: Fix potential null pointer dereference

If key is null in vb2_verify_digest(), we could attempt to dereference
it.  In practice it never is, but for safety's sake we should avoid
the reference.

BUG=chrome-os-partner:32235
BRANCH=none
TEST=VBOOT2=1 make runtests

Change-Id: I5a817e432922ea4c3b439b696cd2f8d988d0fecc
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/219574
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
1 file changed