In standard_element_start(), we were only setting the tree root if the
first element in the file was an <svg> element.  Otherwise we would
simply unref the element at the end of the function.  This caused its
first child to be left with weak reference to a dead parent; the
subsequent call to child.get_parent() would upgrade().unwrap() on the
weak reference, and the unwrap() would panic as the parent is already
dead.

#335