Check that we've got a complete header before accessing its fields.

This patch was prepared by Evgeniy Stepanov (eugenis@chromium.org) and reviewed
at http://codereview.chromium.org/6353010/

BUG=70376
TEST=none
TBR=darin,willchan

Review URL: http://codereview.chromium.org/6347013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72634 0039d316-1c4b-4281-b951-d872f2087c98