Fix crash when element is being inserted "after itself"

tinyxml2.cpp

@@ -911,6 +911,13 @@ XMLNode* XMLNode::InsertAfterChild( XMLNode* afterThis, XMLNode* addThis )
         TIXMLASSERT( false );
         return 0;
     }
+    if ( afterThis == addThis ) {
+        // Current state: BeforeThis -> AddThis -> OneAfterAddThis
+        // Now AddThis must disappear from it's location and then
+        // reappear between BeforeThis and OneAfterAddThis.
+        // So just leave it where it is.
+        return addThis;
+    }
 
     if ( afterThis->_next == 0 ) {
         // The last node or the only node.

xmltest.cpp

@@ -385,6 +385,34 @@ int main( int argc, const char ** argv )
 		doc.Print();
 	}
 
+	{
+		// This test is pre-test for the next one
+		// (where Element1 is inserted "after itself".
+		// This code didn't use to crash.
+		XMLDocument doc;
+		XMLElement* element1 = doc.NewElement("Element1");
+		XMLElement* element2 = doc.NewElement("Element2");
+		doc.InsertEndChild(element1);
+		doc.InsertEndChild(element2);
+		doc.InsertAfterChild(element2, element2);
+		doc.InsertAfterChild(element2, element2);
+	}
+
+	{
+		XMLDocument doc;
+		XMLElement* element1 = doc.NewElement("Element1");
+		XMLElement* element2 = doc.NewElement("Element2");
+		doc.InsertEndChild(element1);
+		doc.InsertEndChild(element2);
+
+		// This insertion "after itself"
+		// used to cause invalid memory access and crash
+		doc.InsertAfterChild(element1, element1);
+		doc.InsertAfterChild(element1, element1);
+		doc.InsertAfterChild(element2, element2);
+		doc.InsertAfterChild(element2, element2);
+	}
+
 	{
 		static const char* test = "<element>Text before.</element>";
 		XMLDocument doc;