Fixes 500 error when user types in wrong group name.

CHANGELOG

@@ -6,6 +6,7 @@ v 6.3.0
   - Show branches list (which branches contains commit) on commit page (Andrew Kumanyaev)
   - Security improvements
   - Added support for GitLab CI 4.0
+  - Fixed issue with 500 error when group did not exist
 
 v 6.2.0
   - Public project pages are now visible to everyone (files, issues, wik, etc.)

app/controllers/groups_controller.rb

@@ -102,7 +102,7 @@ class GroupsController < ApplicationController
 
   # Dont allow unauthorized access to group
   def authorize_read_group!
-    unless projects.present? or can?(current_user, :read_group, @group)
+    unless @group and (projects.present? or can?(current_user, :read_group, @group))
       return render_404
     end
   end