Check payload is not null when decoded. (#444)

* Check payload is not null when decoded.
Fixed "Cannot read property 'nbf' of null"

* Condition on obj !== null for clarity

* Added test for decoding null token

decode.js

@@ -10,7 +10,7 @@ module.exports = function (jwt, options) {
   if(typeof payload === 'string') {
     try {
       var obj = JSON.parse(payload);
-      if(typeof obj === 'object') {
+      if(obj !== null && typeof obj === 'object') {
         payload = obj;
       }
     } catch (e) { }

test/decoding.tests.js

@@ -0,0 +1,12 @@
+var jwt = require('../index');
+var expect = require('chai').expect;
+var atob = require('atob');
+
+describe('decoding', function() {
+
+  it('should not crash when decoding a null token', function () {
+    var decoded = jwt.decode("null");
+    expect(decoded).to.equal(null);
+  });
+
+});