qt: menus: fix memory leak

Indirect leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7f04fac97f41 in operator new(unsigned long) /build/gcc/src/gcc/libsanitizer/asan/asan_new_delete.cpp:99
    #1 0x7f04e0928773 in VLCMenuBar::PopupMenu(intf_thread_t*, bool) ../../modules/gui/qt/menus/menus.cpp:864
    #2 0x7f04e060c348 in DialogsProvider::sendKey(int) ../../modules/gui/qt/dialogs/dialogs_provider.cpp:814
    #3 0x7f04de8cc4be in QObject::event(QEvent*) (/usr/lib/libQt5Core.so.5+0x2e24be)
    #4 0x7f04df367751 in QApplicationPrivate::notify_helper(QObject*, QEvent*) (/usr/lib/libQt5Widgets.so.5+0x15a751)