crash on some SVG pages when svg.in-content.enabled=false

Kathy and I found a web page that consistently causes a NULL pointer dereference during page load in TB 4.5 when svg.in-content.enabled is set to false (e.g., when the security slider is set to the highest position).

We are working on a fix; the problem is related to the fact that elements with the SVG XML namespace are created as regular XML elements when svg.in-content.enabled=false.