SAML verifyResponseSig dereferences nil pointer

[jaym]

The code

	assertion, err := etreeutils.NSSelectOne(response, "urn:oasis:names:tc:SAML:2.0:assertion", "Assertion")
	if err != nil {
		return nil, false, fmt.Errorf("response does not contain an Assertion element")
	}

seems to leave assertion nil when when an element is not found instead of returning an error, causing a panic when it is later used.

I think the errors in #1757 were this panic