Speculative crash fix in LinkStyle

Making sure LinkStyle::loadingFrame() is not null before use.

BUG=400564
TEST=none
R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/452563002

git-svn-id: svn://svn.chromium.org/blink/trunk@179763 bbb929c8-8fbe-4397-9dbb-9b2b20218538