crossystem: Fix null pointer dereference on VMs

Check the result of VbSharedDataRead() before dereferencing it.

BUG=chromium:789276,chromium:819695
BRANCH=none
TEST=make runtests

Change-Id: I1b1cc90bdc2fca61a9aad6b02e8b7e1f6a919797
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/954712
Commit-Ready: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed