ASAN: Fix leak in XWindowTasksModel

Trace:
Direct leak of 24 byte(s) in 3 object(s) allocated from:
    #0 0x544cc0 in operator new(unsigned long) (/home/kfunk/devel/install/kf5/bin/plasmashell+0x544cc0)
    #1 0x7fe4f0e82df9 in TaskManager::XWindowTasksModel::Private::windowInfo(unsigned long long) /home/kfunk/devel/src/kf5/plasma-workspace/libtaskmanager/xwindowtasksmodel.cpp:419:25
    #2 0x7fe4f0e87e69 in TaskManager::XWindowTasksModel::data(QModelIndex const&, int) const /home/kfunk/devel/src/kf5/plasma-workspace/libtaskmanager/xwindowtasksmodel.cpp:646:38
    ...