Fix memory leak in wxXmlNode::operator=().

We must delete all children and attributes in the node being overwritten and
not just the first one of each.

Add a unit test exercising this code to be able to check that valgrind doesn't
report memory leak any more after the fix.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73990 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/xml/xml.cpp

@@ -91,6 +91,18 @@ wxXmlNode::wxXmlNode(const wxXmlNode& node)
 }
 
 wxXmlNode::~wxXmlNode()
+{
+    DoFree();
+}
+
+wxXmlNode& wxXmlNode::operator=(const wxXmlNode& node)
+{
+    DoFree();
+    DoCopy(node);
+    return *this;
+}
+
+void wxXmlNode::DoFree()
 {
     wxXmlNode *c, *c2;
     for (c = m_children; c; c = c2)
@@ -107,14 +119,6 @@ wxXmlNode::~wxXmlNode()
     }
 }
 
-wxXmlNode& wxXmlNode::operator=(const wxXmlNode& node)
-{
-    wxDELETE(m_attrs);
-    wxDELETE(m_children);
-    DoCopy(node);
-    return *this;
-}
-
 void wxXmlNode::DoCopy(const wxXmlNode& node)
 {
     m_type = node.m_type;