Although it's the caller's duty to avoid WSClient re-closing, we
still can handle it.
Fixes the following neofs-node error:
```
panic: close of closed channel
goroutine 98 [running]:
github.com/nspcc-dev/neo-go/pkg/rpc/client.(*WSClient).Close(...)
github.com/nspcc-dev/neo-go@v0.98.3-pre.0.20220321144433-3b639f518ebb/pkg/rpc/client/wsclient.go:120
github.com/nspcc-dev/neofs-node/pkg/morph/subscriber.(*subscriber).Close(0x13)
github.com/nspcc-dev/neofs-node/pkg/morph/subscriber/subscriber.go:108 +0x29
github.com/nspcc-dev/neofs-node/pkg/morph/event.listener.Stop(...)
github.com/nspcc-dev/neofs-node/pkg/morph/event/listener.go:573
created by github.com/nspcc-dev/neofs-node/pkg/innerring.(*Server).Stop
github.com/nspcc-dev/neofs-node/pkg/innerring/innerring.go:285 +0x12f
```
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