Fix panic in integration tests

Closing activationLock only if it's not closed already. This is needed
only because integration tests using docker code directly and doesn't
care about global state.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

api/server/server_linux.go

@@ -95,7 +95,9 @@ func AcceptConnections(job *engine.Job) error {
 	go systemd.SdNotify("READY=1")
 
 	// close the lock so the listeners start accepting connections
-	if activationLock != nil {
+	select {
+	case <-activationLock:
+	default:
 		close(activationLock)
 	}