workspace: Fix infinite loop when finding parent window of dialogs

workspace: Fix infinite loop when finding parent window of dialogs

When a dialog is added to a window while the overview is shown, we get
its parent using get_transient_for() so we can add it to the right
window clone.

If we have multiple layers of dialogs we have to do this recursively
until we find the root ancestor. This case currently results in an
infinite loop: Since parent is always set to the same window, the
while-condition will always be true.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/180