pkg/updateengine: ReceiveStatuses() may get nil signal on shutdown and panic

[invidian]

Originally reported here: coreos/bugs#1987

However, coreos/locksmith@8b0a62e doesn't seem like a correct fix for it, as you can avoid getting nil signal completely, if you wait for receiver loop to shut down, e.g. by waiting for the final nil value from given ch on the process spawning ReceiveStatuses() goroutine.

Also related: godbus/dbus#271

[invidian]

Actually, this does not affect agent, as it always drains the receive channel, but it's important to notice when consuming pkg/updateengine directly, so we should have unit tests for this.

[invidian]

This also happens when you close the client before stopping the receiver routine, so clearly a bug.