make Attachment#make_childless safe to call more often

i.e. if it's already a child, or it has no children, and you don't
want to check those beforehand

Change-Id: Ie0568c980f888b1e39f64aa0108033222887b2b6
Reviewed-on: https://gerrit.instructure.com/97612
Tested-by: Jenkins
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

app/models/attachment.rb

@@ -1213,7 +1213,9 @@ class Attachment < ActiveRecord::Base
   end
 
   def make_childless(preferred_child = nil)
+    return if root_attachment_id
     child = preferred_child || children.take
+    return unless child
     raise "must be a child" unless child.root_attachment_id == id
     child.root_attachment_id = nil
     child.filename = filename if filename