Merge pull request #3610 from Random-Liu/fix-containerd-panic

Fix potential containerd panic during graceful shutdown.

cmd/containerd/command/main_unix.go

@@ -58,6 +58,7 @@ func handleSignals(ctx context.Context, signals chan os.Signal, serverC chan *se
 					}
 					server.Stop()
 					close(done)
+					return
 				}
 			}
 		}