Fixing crash in animated CSS clipping

Adding a null pointer check to verify the existence
of a clipping shape before trying to use it.

BUG=396593
TEST=https://cluster-fuzz.appspot.com/testcase?key=5487631620112384

Review URL: https://codereview.chromium.org/416543004

git-svn-id: svn://svn.chromium.org/blink/trunk@178863 bbb929c8-8fbe-4397-9dbb-9b2b20218538