update_engine: Fix an invalid memory access in OmahaRequestAction::TransferComplete

The bug only happens when the omaha response is invalid, but still is
very dangerous and could crash the update engine.

BUG=none
TEST=This unittest pass on veyron_minnie

Change-Id: I86f3f802d78815ef09eaa2dd96a288bdf28882a8
Reviewed-on: https://chromium-review.googlesource.com/1164559


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>