verity: handle trees with an odd node count correctly

This fixes a bug we were seeing when setting root_depth=3.

BUG=none
TEST=Ran unittests.

Change-Id: I7241ccd97102638e9f003a694280ca0b53f317b9

R=wad@chromium.org

Review URL: http://codereview.chromium.org/6742001
2 files changed