firmware: Die on zero size errors vb2api_get_firmware_size() BUG=none BRANCH=none TEST=make runtests -j8 Signed-off-by: Jakub Czapiga <jacz@semihalf.com> Disallow-Recycled-Builds: test-failures Change-Id: I9c22627410836906a84c387377facc778dd77f3b Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3986523 Tested-by: Jakub Czapiga <czapiga@google.com> Reviewed-by: Julius Werner <jwerner@chromium.org> Commit-Queue: Jakub Czapiga <czapiga@google.com>