Fail when client use not supported protocol handler

## Error
***
```
CALLBACK FAILED: /usr/local/share/lua/5.1/websocket/server_ev.lua:104: attempt to index field '?' (a nil value)
```
## Fix
***
```
clients[protocol][self] = nil
```
↓
```
if clients[protocol] ~= nil and clients[protocol][self] ~= nil then
  clients[protocol][self] = nil
end
```