Fix issue #1048: Add null pointer check in XMLDocument::DeleteNode (#1057)

- Add null pointer check in XMLDocument::DeleteNode() method
- Add test case to verify null pointer handling
- Prevents segmentation fault when calling DeleteNode(nullptr)

tinyxml2.cpp

@@ -2347,9 +2347,12 @@ static FILE* callfopen( const char* filepath, const char* mode )
     return fp;
 }
 
-void XMLDocument::DeleteNode( XMLNode* node )	{
+void XMLDocument::DeleteNode( XMLNode* node )	{   
     TIXMLASSERT( node );
     TIXMLASSERT(node->_document == this );
+    if(node == 0) {
+        return; // check for null pointer
+    }
     if (node->_parent) {
         node->_parent->DeleteChild( node );
     }