Check for NULL after Frame lookup in findFrameByName

This function was crashing if the FrameTree name lookup failed to find
a matching frame.

TBR=japhet@chromium.org
BUG=383845

Review URL: https://codereview.chromium.org/331633004

git-svn-id: svn://svn.chromium.org/blink/trunk@176058 bbb929c8-8fbe-4397-9dbb-9b2b20218538