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>
(cherry picked from commit c71747571458bf992c729c483c9509e8182d630b)

Docker-DCO-1.1-Signed-off-by: Jessie Frazelle <jess@docker.com> (github: jfrazelle)

Docker-DCO-1.1-Signed-off-by: Jessie Frazelle <princess@docker.com> (github: jfrazelle)
diff --git a/api/server/server_linux.go b/api/server/server_linux.go
index fff803d..a931157 100644
--- a/api/server/server_linux.go
+++ b/api/server/server_linux.go
@@ -95,7 +95,9 @@
 	go systemd.SdNotify("READY=1")
 
 	// close the lock so the listeners start accepting connections
-	if activationLock != nil {
+	select {
+	case <-activationLock:
+	default:
 		close(activationLock)
 	}