Fix a NULL ptr dereference in FELighting. This only occurs with the "GPU-Accelerated SVG Filters" flag enabled in about:flags.

BUG=314943

Review URL: https://codereview.chromium.org/70983003

git-svn-id: svn://svn.chromium.org/blink/trunk@162161 bbb929c8-8fbe-4397-9dbb-9b2b20218538