Improve coverage of rollback_index.c

BUG=chromium-os:38139
BRANCH=none
TEST=make runtests

Change-Id: I21b62b5dd3fc6037f54f7c3bac768c2b67a4c12d
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/41859
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
4 files changed