Fix wxXmlNode self-assignment.

Don't lose the node contents if it's assigned to itself.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73992 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/xml/xml.cpp

@@ -97,8 +97,12 @@ wxXmlNode::~wxXmlNode()
 
 wxXmlNode& wxXmlNode::operator=(const wxXmlNode& node)
 {
-    DoFree();
-    DoCopy(node);
+    if ( &node != this )
+    {
+        DoFree();
+        DoCopy(node);
+    }
+
     return *this;
 }