Fix an NPE in SVGFEImageElement

SVGFEImageElement::notifyFinished did not properly handle having a
null parent. This crash would not happen if isSVGFilterElement(parent)
had been used instead of isSVGFilterElement(*parent). This patch
refactors this code to be robust to a null parent and adds a CF-based
test to prove it works.

BUG=461151

Review URL: https://codereview.chromium.org/972903002

git-svn-id: svn://svn.chromium.org/blink/trunk@191263 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed