Fix potential leak in nodehelper

ASAN trace:
Indirect leak of 48 byte(s) in 2 object(s) allocated from:
    #0 0x52a000 in operator new(unsigned long) (/home/kfunk/devel/install/kf5/bin/kmail+0x52a000)
    #1 0x7fbadd2548fb in MimeTreeParser::NodeHelper::removeTempFiles() /home/kfunk/devel/src/kf5/messagelib/mimetreeparser/src/nodehelper.cpp:340:27
    #2 0x7fbaddf8b7ac in MessageViewer::ViewerPrivate::parseContent(KMime::Content*) /home/kfunk/devel/src/kf5/messagelib/messageviewer/src/viewer/viewer_p.cpp:904:18
    #3 0x7fbaddf8b1c6 in MessageViewer::ViewerPrivate::displayMessage() /home/kfunk/devel/src/kf5/messagelib/messageviewer/src/viewer/viewer_p.cpp:885:5
    ...