rsvg_standard_element_start(): Remove unnecessary check
rsvg_standard_element_start(): Remove unnecessary check

No creator function should ever return a NULL node, so assert that and
remove the check.

For elements of unknown type, we already turn them into groups in
get_node_creator_for_element_name().

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg