CTTE: RenderSVGInlineText always has a Text node.
<https://webkit.org/b/121409>

Reviewed by Antti Koivisto.

This renderer is never anonymous and always has a corresponding Text node.
Replaced node() with a textNode() reference getter.

Only one null check was harmed in the making of this patch.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155837 268f45cc-cd09-0410-ab3c-d52691b4dbfc