If SendWorker queue is full, check if that thread is still alive.
Otherwise it would deadlock on putting an entry to that queue.
This also requires that SendWorker must ensure that the main thread
isn't currently waiting for queue space when it fails. We can do this by
simply removing an entry from a queue - so on the next iteration
SendWorker would be already dead and main thread would notice it.
Getting an entry from queue in such (error) situation is harmless,
because other checks will notice it's an error condition.
FixesQubesOS/qubes-issues#1359
0c476f014d