Add a missing null check in HistoryController::createItemTree

When pushState() is called, sometimes a history item doesn't exist for that
frame. In that case, we blindly try to acces the null HistoryItem*.

Unfortunately, I haven't been able to determine the exact case where this
occurs, so no new test.

BUG=321608

Review URL: https://codereview.chromium.org/79223002

git-svn-id: svn://svn.chromium.org/blink/trunk@162408 bbb929c8-8fbe-4397-9dbb-9b2b20218538