Fix a crash occuring when closing a non-connected websocket

[scottinet]

Description

This PR fixes a segfault case occuring when closing an unconnected WebSocket instance.

How to test

Run this Go program:

package main

import (
	"github.com/kuzzleio/sdk-go/protocol/websocket"
)

func main() {
	c := websocket.NewWebSocket("kuzzle", nil)
	c.Close()
}

Without this PR: segfault
With this PR: 👍

[codecov-io]

Codecov Report

Merging #239 into 1-dev will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##            1-dev     #239      +/-   ##
==========================================
- Coverage   87.99%   87.95%   -0.04%     
==========================================
  Files         244      244              
  Lines        4491     4493       +2     
==========================================
  Hits         3952     3952              
- Misses        487      489       +2     
  Partials       52       52

Impacted Files	Coverage Δ
protocol/websocket/web_socket.go	9.22% <0%> (-0.06%)	⬇️

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