(loadgen) fix regression, when connections fail loadgen should not crash

lib/load_gen/common.js

@@ -67,7 +67,7 @@ exports.genAssertionAndVerify = function(cfg, user, ctx, email, audience, cb) {
     try {
       if (!typeof JSON.parse(r.body) === 'object') throw 'bogus response';
     } catch(e) {
-      return cb(e.toString() + " - " + r.body);
+      return cb(e.toString() + (r ? (" - " + r.body) : ""));
     }
 
     var assertion = crypto.getAssertion({