rpc: avoid panic during request after WS connection is closed
Fix the following panic:
```
panic: assignment to entry in nil map
goroutine 131 [running]:
github.com/nspcc-dev/neo-go/pkg/rpc/client.(*WSClient).registerRespChannel(0xc00033c240, 0x0, 0xc00003e2a0)
/home/denis/go/pkg/mod/github.com/nspcc-dev/neo-go@v0.98.2/pkg/rpc/client/wsclient.go:244 +0x96
github.com/nspcc-dev/neo-go/pkg/rpc/client.(*WSClient).makeWsRequest(0xc00033c240, 0xc002080000)
/home/denis/go/pkg/mod/github.com/nspcc-dev/neo-go@v0.98.2/pkg/rpc/client/wsclient.go:264 +0x69
github.com/nspcc-dev/neo-go/pkg/rpc/client.(*Client).performRequest(0xc00033c240, {0xc9f173, 0xd}, {{0x13d09d0, 0x0, 0x0}}, {0xb44120, 0xc00147a000})
/home/denis/go/pkg/mod/github.com/nspcc-dev/neo-go@v0.98.2/pkg/rpc/client/client.go:186 +0x15d
github.com/nspcc-dev/neo-go/pkg/rpc/client.(*Client).GetBlockCount(0xc001fb5440)
/home/denis/go/pkg/mod/github.com/nspcc-dev/neo-go@v0.98.2/pkg/rpc/client/rpc.go:73 +0x69
...
```
b1f74b549d