Fix the sample size check in while determining degree of freedom.
Fix the sample size check in while determining degree of freedom.

BUG=
R=qyearsley@chromium.org, robertocn@chromium.org

Review URL: https://codereview.chromium.org/1358653003 .

luci/recipes-py,luci/recipes-py,shishkander/recipes-py,shishkander/recipes-py