Add a null check for m_client in FrameLoader::detachClient.

BUG=363099
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/262853002

git-svn-id: svn://svn.chromium.org/blink/trunk@173123 bbb929c8-8fbe-4397-9dbb-9b2b20218538