Some SVGs define a mask but an invalid mask frame. Check to make sure
we have a `maskFrame` that isn't null before trying to use it.
This resolves#1034
AnimVal() is a DOM getter, and it flushes animations, which we don't want
in GetBBox() which is called from display list building cide and
FrameLayerBuilder.
MozReview-Commit-ID: DWgm7wAV7C0