The invalid parent condition can actually happen because PHP's DOM is
allows to get children of e.g. attributes; something normally not
possible.
Closes GH-16597.
Lower branches suffer from this as well but we cannot change the
behaviour there.
We also add NULL checks to check for allocation failure.
Closes GH-15014.
I don't know why this code was here in the first place, it is present
since the initial implementation. It doesn't make sense because:
1. It would require updating the refcounts if the document wasn't
actually already set.
2. We enforce that the document is the same as the target document by
this point, so setting the tree is pointless.
This introduces a new helper php_dom_create_nullable_object() that does
the NULL check and puts NULL in return_value. Otherwise it runs
php_dom_create_object(). This deduplicates a bit of code.
