device: remove QueueInboundElement leak with stopped peers

This is particularly problematic on mobile,
where there is a fixed number of elements.
If most of them leak, it'll impact performance;
if all of them leak, the device will permanently deadlock.

I have a test that detects element leaks, which is how I found this one.
There are some remaining leaks that I have not yet tracked down,
but this is the most prominent by far.

I will commit the test when it passes reliably.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/receive.go

@@ -199,6 +199,8 @@ func (device *Device) RoutineReceiveIncoming(IP int, bind conn.Bind) {
 				if device.addToInboundAndDecryptionQueues(peer.queue.inbound, device.queue.decryption, elem) {
 					buffer = device.GetMessageBuffer()
 				}
+			} else {
+				device.PutInboundElement(elem)
 			}
 			peer.queue.RUnlock()