Fix GH-14702: DOMDocument::xinclude() crash

[nielsdos]

The xinclude code from libxml removes the fallback node, but the fallback node is still reference via $fallback. The solution is to detach the nodes that are going to be removed in advance.

[devnexen]

I guess no choice, hopefully no real impact perf wise

[nielsdos]

I guess no choice, hopefully no real impact perf wise

Yeah I don't see a better way, I tried putting all the cheap checks first.

We're actually already doing a tree traversal to get rid of the xinclude fake start and end nodes.
I suppose the main bottleneck here is data cache utilisation...

In an ideal world we should combine the php_dom_remove_xinclude_nodes into the code I added such that the performance impact is minimal, but that requires some refactoring as php_dom_remove_xinclude_nodes is recursive instead of iterative, and should be left for master.

[nielsdos]

In an ideal world we should combine the php_dom_remove_xinclude_nodes into the code I added such that the performance impact is minimal, but that requires some refactoring as php_dom_remove_xinclude_nodes is recursive instead of iterative, and should be left for master.

Err scratch that, the reference stripping must happen before xinclude, but the pre-existing removal of the fake nodes must happen after the xinclude... Annoying....