Fix crash on expired certificates

[alubbe]

I've run into an error that's crashed our monitoring service. In short, when trying to check an https service which runs an expired certificate, nodejs doesn't create socket.ssl (at least in node v13), and thus oscp proceeds to throw an error. In our case, we're using the https module with ocsp's Agent, which produces the following error:

node_modules/ocsp/lib/ocsp/agent.js:70
  var cert = socket.ssl.getPeerCertificate(true);
                        ^

TypeError: Cannot read property 'getPeerCertificate' of null
    at Agent.handleOCSPResponse (node_modules/ocsp/lib/ocsp/agent.js:70:25)
    at TLSSocket.<anonymous> (node_modules/ocsp/lib/ocsp/agent.js:52:17)
    at TLSSocket.emit (events.js:215:7)
    at TLSSocket._finishInit (_tls_wrap.js:794:8)
    at TLSWrap.ssl.onhandshakedone (_tls_wrap.js:608:12)

This can be fixed by moving the relevant code into the try/catch statement that already exists in Agent.prototype.handleOCSPResponse, which then allows https to display the actual error properly:

Error: certificate has expired
    at TLSSocket.onConnectSecure (_tls_wrap.js:1321:34)
    at TLSSocket.emit (events.js:215:7)
    at TLSSocket._finishInit (_tls_wrap.js:794:8)
    at TLSWrap.ssl.onhandshakedone (_tls_wrap.js:608:12)

[alubbe]

Travis reports one failing test, but I don't think it should block this PR because it also fails on master:

 1 failing

  1) OCSP Stapling Provider .check() should validate google.com:

     Uncaught Error: Bad OCSP response status: unauthorized