fix: Cannot read property 'Symbol(TIMER::READ_TIMER)' of null

[nodejh]

当使用 SocksProxyAgent 做代理的时候，如果 new SocksProxyAgent('socks://127.0.0.1:3000') 中的 address 无法访问，则 httpx 内部会抛错，并且这个错误无法被正常捕获。

$ npm run test

> httpx@2.2.3 test /Users/jh/inbox/github/httpx
> mocha --reporter spec --timeout 3000 test/*.test.js



  httpx
(node:14882) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'Symbol(TIMER::READ_TIMER)' of null
    at ClientRequest.rejected (/Users/jh/inbox/github/httpx/lib/index.js:98:25)
    at ClientRequest.emit (events.js:196:13)
    at onerror (/Users/jh/inbox/github/httpx/node_modules/agent-base/dist/src/index.js:114:21)
    at callbackError (/Users/jh/inbox/github/httpx/node_modules/agent-base/dist/src/index.js:133:17)
    at processTicksAndRejections (internal/process/task_queues.js:89:5)
(node:14882) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:14882) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

  it.only('should throw an error', async function () {
    try {
      // socks://127.0.0.1:3000 is an invalid socks proxy address.
      await make(server)('/', { agent: new SocksProxyAgent('socks://127.0.0.1:3000') });
      assert.fail('should not run here');
    } catch (error) {
      const port = server.address().port;
      assert.equal(error.message, `connect ECONNREFUSED 127.0.0.1:3000GET http://127.0.0.1:${port}/ failed.`);
    }
  });

[nodejh]

当 socks 地址有误时，socket 就无法连接上，startResponseTimer 也就不会执行，所以这里 request.socket 也就是 null，所以需要判断一下。

[coveralls]

Coverage remained the same at 78.621% when pulling 0600643 on nodejh:master into bcc8ddb on JacksonTian:master.