Add null-check for likely cause of a top crasher in HistoryController

It appears that if we load a javascript into a newly created iframe, we can
create the new frame without creating a new history item (neither real nor
dummy). In that case, HistoryController::restoreDocumentState may crash,
because it assumes m_currentItem is non-null.

BUG=301840
TEST=Haven't reduced one yet, will land one in a followup patch ASAP
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25384002

git-svn-id: svn://svn.chromium.org/blink/trunk@158550 bbb929c8-8fbe-4397-9dbb-9b2b20218538