prevent panic when session is nil

Signed-off-by: Jörn Friedrich Dreyer <jfd@butonic.de>

services/idp/pkg/backends/cs3/identifier/session.go

@@ -36,5 +36,8 @@ type cs3Session struct {
 
 // User returns the cs3 user of the session
 func (s *cs3Session) User() *cs3user.User {
+	if s == nil {
+		return nil
+	}
 	return s.u
 }