Invalid Base64 encoding results in "TypeError: Cannot read property 'documentElement' of null"

[markstos]

If a response is received with invalid Base64 encoding, and the response is attempted to validated, instead of throwing a reasonable error, validatePostResponse will fail with this error:

TypeError: Cannot read property 'documentElement' of null

The issue could be resolved with better error checking. If this fails:

var doc = new xmldom.DOMParser().parseFromString(xml);

Then "doc" will be null. We can test for that and return error that the either the response was not Base64 encoded, or the XML was malformed. By testing xml on the line above:

var xml = new Buffer(container.SAMLResponse, 'base64').toString('utf8');  

We may be able to differiate between the case when the base64 encoding was broken and the XML was malformed.

This related to #118, which also suggested that this error could handled better.