Don't call grpc stop before server initialized

When receiving a stop signal, it is possible that the GRPC server has
not yet been iniitialized. Verify that is has been before trying to stop
the server.

(cherry picked from commit 0545aaf)

gocsi.go

@@ -320,7 +320,9 @@ func (sp *StoragePlugin) Serve(ctx context.Context, lis net.Listener) error {
 // errors.
 func (sp *StoragePlugin) Stop(ctx context.Context) {
 	sp.stopOnce.Do(func() {
-		sp.server.Stop()
+		if sp.server != nil {
+			sp.server.Stop()
+		}
 		log.Info("stopped")
 	})
 }
@@ -330,7 +332,9 @@ func (sp *StoragePlugin) Stop(ctx context.Context) {
 // pending RPCs are finished.
 func (sp *StoragePlugin) GracefulStop(ctx context.Context) {
 	sp.stopOnce.Do(func() {
-		sp.server.GracefulStop()
+		if sp.server != nil {
+			sp.server.GracefulStop()
+		}
 		log.Info("gracefully stopped")
 	})
 }