Fix leak in messagepartrendererfactory

ASAN trace:
Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x52a000 in operator new(unsigned long) (/home/kfunk/devel/install/kf5/bin/kmail+0x52a000)
    #1 0x7fc9353b3eca in MessageViewer::MessagePartRendererFactoryPrivate::initialize_builtin_renderers() /home/kfunk/devel/src/kf5/messagelib/messageviewer/src/messagepartthemes/default/messagepartrendererfactory.cpp:109:43
    #2 0x7fc9353b3e4e in MessageViewer::MessagePartRendererFactoryPrivate::setup() /home/kfunk/devel/src/kf5/messagelib/messageviewer/src/messagepartthemes/default/messagepartrendererfactory.cpp:57:9
    #3 0x7fc9353b45a2 in MessageViewer::MessagePartRendererFactory::renderersForPart(QMetaObject const*, QSharedPointer<MimeTreeParser::MessagePart> const&) const /home/kfunk/devel/src/kf5/messagelib/messageviewer/src/messagepartthemes/default/messagepartrendererfactory.cpp:152:8
    #4 0x7fc935398f06 in MessageViewer::DefaultRendererPrivate::renderWithFactory(QMetaObject const*, QSharedPointer<MimeTreeParser::MessagePart> const&, MessageViewer::HtmlWriter*) /home/kfunk/devel/src/kf5/messagelib/messageviewer/src/messagepartthemes/default/defaultrenderer.cpp:855:37
    ...