Fix bug introduced in 610048179c64.
Fix bug introduced in 610048179c64.

It resulted in a HTTP 500 in task that had a parent.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.appspot.com/196650043

pombreda/swarming,luci/luci-py,pombreda/swarming,madecoste/swarming,luci/luci-py,luci/luci-py,madecoste/swarming,pombreda/swarming,madecoste/swarming,luci/luci-py,madecoste/swarming,pombreda/swarming