saml connector: fix nil pointer on validate saml (#3793)

Signed-off-by: Siarhei Haurylau <siarhei.haurylau@point-devel.com>

connector/saml/saml.go

@@ -597,6 +597,9 @@ func verifyResponseSig(validator *dsig.ValidationContext, data []byte) (signed [
 	}
 
 	response := doc.Root()
+	if response == nil {
+		return nil, false, fmt.Errorf("parse document: empty root")
+	}
 	transformedResponse, err := validator.Validate(response)
 	if err == nil {
 		// Root element is verified, return it.
@@ -609,7 +612,7 @@ func verifyResponseSig(validator *dsig.ValidationContext, data []byte) (signed [
 	//
 	// TODO: Only select from child elements of the root.
 	assertion, err := etreeutils.NSSelectOne(response, "urn:oasis:names:tc:SAML:2.0:assertion", "Assertion")
-	if err != nil {
+	if err != nil || assertion == nil {
 		return nil, false, fmt.Errorf("response does not contain an Assertion element")
 	}
 	transformedAssertion, err := validator.Validate(assertion)