[SVG] Fix crash when <animate> is instantiated by <use> on <use>

The patch adds missing null checks to SVGAnimatedNewPropertyAnimator.

BUG=343081

Review URL: https://codereview.chromium.org/164183005

git-svn-id: svn://svn.chromium.org/blink/trunk@167159 bbb929c8-8fbe-4397-9dbb-9b2b20218538