BugFix: Fail gracefully when SAML Response is invalid. Fixes #238

Before, given junk input, validatePostResponse would fail with:

   TypeError: Cannot read property 'documentElement' of null

Now we'll fail with:

    SAMLResponse is not valid base64-encoded XML

To make this work, we primarily just needed to as a simple additional error check,
but to throw the error properly, we needed to move a bit of logic into
a nearby promise, but keeping some variable definitions in the outer
scope where they continue to be expected.

lib/passport-saml/saml.js

@@ -512,15 +512,23 @@ SAML.prototype.validateSignature = function (fullXml, currentNode, cert) {
 
 SAML.prototype.validatePostResponse = function (container, callback) {
   var self = this;
-  var xml = new Buffer(container.SAMLResponse, 'base64').toString('utf8');
-  var doc = new xmldom.DOMParser().parseFromString(xml);
 
-  var inResponseTo = xpath(doc, "/*[local-name()='Response']/@InResponseTo");
-  if(inResponseTo){
-    inResponseTo = inResponseTo.length ? inResponseTo[0].nodeValue : null;
-  }
+  var xml, doc, inResponseTo;
 
   Q.fcall(function(){
+    xml = new Buffer(container.SAMLResponse, 'base64').toString('utf8');
+    doc = new xmldom.DOMParser({
+    }).parseFromString(xml);
+
+    if (!doc.hasOwnProperty('documentElement'))
+      throw new Error('SAMLResponse is not valid base64-encoded XML');
+
+    inResponseTo = xpath(doc, "/*[local-name()='Response']/@InResponseTo");
+
+    if(inResponseTo){
+      inResponseTo = inResponseTo.length ? inResponseTo[0].nodeValue : null;
+    }
+
     if(self.options.validateInResponseTo){
       if (inResponseTo) {
         return Q.ninvoke(self.cacheProvider, 'get', inResponseTo)