Xi: Fix segfault when removing slave

Kept running into a segfault somewhere in GetDeviceUse whenever I tried
to remove a device. This happened on manual removal or on server
shutdown.

Now in XISendDeviceHierarchyEvent we don't try to read properties of
recently removed devices.
4 jobs for segfault_when_removing_masters in 6 minutes and 21 seconds (queued for 2 seconds)
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