According to the DOM standard, elements may only contain element, text,
processing instruction and comment nodes[1]. It is also specified that
a HierarchyRequestError should be thrown if a document is to be
inserted[2]. We follow that standard, and prevent the use-after-free
this way.
[1] <https://dom.spec.whatwg.org/#node-trees>
[2] <https://dom.spec.whatwg.org/#mutation-algorithms>
Closes GH-6765.
a08847ab39