bugfix mentioned in #45 don't access a proptery that doesn't exist

lib/core.js

@@ -350,7 +350,7 @@ function getOrCreateDomainCertificate(args, defaults, handlers) {
   }
 
   return fetchFromConfigLiveDir(args).then(function (certs) {
-    var halfLife = (certs.expiresAt - certs.issuedAt) / 2;
+    var halfLife = certs && ((certs.expiresAt - certs.issuedAt) / 2);
 
     if (!certs || (Date.now() - certs.issuedAt) > halfLife) {
       // There is no cert available