Fix memory leak in FontLoader::loadFontFromXml

Tried building with -DCMAKE_CXX_FLAGS='-fsanitize=address -fsanitize-recover=address'
and this was one of the reported leaks.

components/fontloader/fontloader.cpp

@@ -630,6 +630,7 @@ namespace Gui
             }
 
             MyGUI::ResourceManager::getInstance().loadFromXmlNode(copy, _file, _version);
+            delete copy;
         }
     }