Fix crash due to crosss-thread list access.

I've seen it happen that item points to a different QueueItem after the
lock was unlocked here, which I didn't think was possible.  Backtraces
suggest that when problems happen _queue is being sorted in another
thread, and perhaps this is not allowed (I couldn't find any conclusive
proof in any documentation).

There is also a potential problem with the newly-added zombify() method,
which takes the lock and clears the list.

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