embed: fix nil pointer dereference when stopServer

Since v3.4.25, ETCD server introduces http-only urls flag to expose
gRPC-only endpoints. When user enables this feature, the stopServer will
panic during terminating. If the server is leader, it won't have chance
to transfer the leadership.

```
Jul 07 14:43:04  etcd[11502]: received terminated signal, shutting down...
Jul 07 14:43:04  etcd[11502]: WARNING: 2023/07/07 14:43:04 grpc: addrConn.createTransport failed to connect to {0.0.0.0:2379  <nil> 0 <nil>}. Err :connection error: desc = "transport: Error while dialing dial tcp 0.0.0.0:2379: connect: connection refused". Reconnecting...Jul 07 14:43:04  etcd[11502]: WARNING: 2023/07/07 14:43:04 grpc: addrConn.createTransport failed to connect to {0.0.0.0:2379  <nil> 0 <nil>}. Err :connection error: desc = "transport: Error while dialing dial tcp 0.0.0.0:2379: connect: connection refused". Reconnecting...
Jul 07 14:43:04  etcd[11502]: panic: runtime error: invalid memory address or nil pointer dereference                                                                                                                                                                           Jul 07 14:43:04  etcd[11502]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x130 pc=0x9ccd45]
Jul 07 14:43:04  etcd[11502]: goroutine 225 [running]:
Jul 07 14:43:04  etcd[11502]: google.golang.org/grpc.(*Server).Stop(0x0)
Jul 07 14:43:04  etcd[11502]:         /home/fuwei/go/pkg/mod/google.golang.org/grpc@v1.26.0/server.go:1390 +0x45
Jul 07 14:43:04  etcd[11502]: go.etcd.io/etcd/embed.stopServers.func1()
Jul 07 14:43:04  etcd[11502]:         /home/fuwei/go/src/go.etcd.io/etcd/embed/etcd.go:431 +0x3c
Jul 07 14:43:04  etcd[11502]: go.etcd.io/etcd/embed.stopServers({0x115a558, 0xc000278b70}, 0xc00024f248)
Jul 07 14:43:04  etcd[11502]:         /home/fuwei/go/src/go.etcd.io/etcd/embed/etcd.go:438 +0x7d
Jul 07 14:43:04  etcd[11502]: go.etcd.io/etcd/embed.(*Etcd).Close(0xc0004d6600)
Jul 07 14:43:04  etcd[11502]:         /home/fuwei/go/src/go.etcd.io/etcd/embed/etcd.go:392 +0x835
Jul 07 14:43:04  etcd[11502]: go.etcd.io/etcd/pkg/osutil.HandleInterrupts.func1()
Jul 07 14:43:04  etcd[11502]:         /home/fuwei/go/src/go.etcd.io/etcd/pkg/osutil/interrupt_unix.go:70 +0x284
Jul 07 14:43:04  etcd[11502]: created by go.etcd.io/etcd/pkg/osutil.HandleInterrupts
Jul 07 14:43:04  etcd[11502]:         /home/fuwei/go/src/go.etcd.io/etcd/pkg/osutil/interrupt_unix.go:53 +0xce
Jul 07 14:43:04  systemd[1]: etcd.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
```

Signed-off-by: Wei Fu <fuweid89@gmail.com>

embed/etcd.go

@@ -426,9 +426,13 @@ func (e *Etcd) Close() {
 func stopServers(ctx context.Context, ss *servers) {
 	shutdownNow := func() {
 		// first, close the http.Server
-		ss.http.Shutdown(ctx)
+		if ss.http != nil {
+			ss.http.Shutdown(ctx)
+		}
 		// then close grpc.Server; cancels all active RPCs
-		ss.grpc.Stop()
+		if ss.grpc != nil {
+			ss.grpc.Stop()
+		}
 	}
 
 	// do not grpc.Server.GracefulStop with TLS enabled etcd server