waylandserver: don't emit windowRemoved if it's not been added yet

This can happen when a client destroys the window before it ever got ready for painting. What prompted this MR is that it happens 100% of the time when Maliit is enabled but not used (-> text field focused with the mouse); KWin has a window in that case but never actually shows it, leading to windowRemoved being emitted without windowAdded being emitted before, which leads to crashes.