Check for null renderer in SVGFEImageElement::notifyFinished()

This bug was found while fixing crbug.com/257648. Other callers of
RenderSVGResource::markForLayoutAndParentResourceInvalidation check for
a null renderer(); we were just missing that check in
SVGFEImageElement::notifyFinished().

No new test as this is a trivial null pointer crash.

Review URL: https://chromiumcodereview.appspot.com/22909006

git-svn-id: svn://svn.chromium.org/blink/trunk@156016 bbb929c8-8fbe-4397-9dbb-9b2b20218538