Fix phpGH-17040: SimpleXML's unset can break DOM objects

Don't free the underlying nodes if we still have objects pointing to
them, otherwise the objects are left with a NULL node pointer.

Closes phpGH-17046.

Author: nielsdos

NEWS

@@ -8,6 +8,9 @@ PHP                                                                        NEWS
 - Iconv:
   . Fixed bug GH-17047 (UAF on iconv filter failure). (nielsdos)
 
+- SimpleXML:
+  . Fixed bug GH-17040 (SimpleXML's unset can break DOM objects). (nielsdos)
+
 - Streams:
   . Fixed bug GH-17037 (UAF in user filter when adding existing filter name due
     to incorrect error handling). (nielsdos)