plugins: don't panic on Close if PluginServer nil

Signed-off-by: Laura Brehm <laurabrehm@hey.com>
(cherry picked from commit 9c4480604e)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

cli-plugins/socket/socket.go

@@ -95,6 +95,9 @@ func (pl *PluginServer) Addr() net.Addr {
 //
 // The error value is that of the underlying [net.Listner.Close] call.
 func (pl *PluginServer) Close() error {
+	if pl == nil {
+		return nil
+	}
 	logrus.Trace("Closing plugin server")
 	// Close connections first to ensure the connections get io.EOF instead
 	// of a connection reset.

cli-plugins/socket/socket_test.go

@@ -117,6 +117,18 @@ func TestPluginServer(t *testing.T) {
 		assert.NilError(t, err, "failed to dial returned server")
 		checkDirNoNewPluginServer(t)
 	})
+
+	t.Run("does not panic on Close if server is nil", func(t *testing.T) {
+		var srv *PluginServer
+		defer func() {
+			if r := recover(); r != nil {
+				t.Errorf("panicked on Close")
+			}
+		}()
+
+		err := srv.Close()
+		assert.NilError(t, err)
+	})
 }
 
 func checkDirNoNewPluginServer(t *testing.T) {