Mir sends resize events for invalid windows

[Alan Griffiths]

install miral-examples vlc

[terminal 1]
$ miral-shell

[terminal 2]
$ miral-xrun vlc

Right click a few times in rapid succession.

Expect: nothing bad
Actual: [terminal 1] Aborted (core dumped)

A bit of investigation shows we abort while processing a resize event in DecorationProvider::handle_event(): when we call mir_window_apply_spec() it aborts because the window is invalid.