Fix GH-12929: SimpleXMLElement with stream_wrapper_register can segfault

Move SimpleXML invalidation code after node checks

This is safe, i.e. the tree hasn't been modified yet, bec

Fix GH-12929: SimpleXMLElement with stream_wrapper_register can segfault

Move SimpleXML invalidation code after node checks

This is safe, i.e. the tree hasn't been modified yet, because either we
didn't call a libxml modification function yet, or xmlNewChild is called
with a NULL pointer, which makes it bail out and return NULL.

Closes GH-12947.

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