Crash in SVGSVGElement::currentViewBoxRect.

Crash in SVGSVGElement::currentViewBoxRect.
https://bugs.webkit.org/show_bug.cgi?id=77121

Reviewed by Nikolas Zimmermann.

Source/WebCore:

Symbols shouldn't be rendered. Revert the ASSERT
from r105513 into a hard check.

Test: svg/custom/symbol-viewport-element-crash.svg

* svg/SVGLengthContext.cpp:
(WebCore::SVGLengthContext::determineViewport):

LayoutTests:

* svg/custom/symbol-viewport-element-crash-expected.txt: Added.
* svg/custom/symbol-viewport-element-crash.svg: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106036 268f45cc-cd09-0410-ab3c-d52691b4dbfc