bisect-kit: fix exception in prepare_chroot()

BUG=None
TEST=None

Change-Id: I7b09defdf1979e4931cf508d2867ba4d2844108b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2844609
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Auto-Submit: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Zheng-Jie Chang <zjchang@chromium.org>
1 file changed