Fixed potential error when sorting members without role (#10053).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@8863 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/principal.rb

@@ -50,7 +50,9 @@ class Principal < ActiveRecord::Base
   end
 
   def <=>(principal)
-    if self.class.name == principal.class.name
+    if principal.nil?
+      -1
+    elsif self.class.name == principal.class.name
       self.to_s.downcase <=> principal.to_s.downcase
     else
       # groups after users