Authorize.net silentpost transactions cannot be edited due to invalid net_amount

This is experienced as a 4.4 regression but I believe the data problem preceeds 4.4.x but extra validation in 4.4.x means people can no longer edit (e.g custom fields or notes or receipts) on these transactions as they get an error :

Get "The sum of fee amount and net amount must be equal to total amount" error when try to save

CRM-9906 touches on this. Basically net_amount is set to 0 by the IPN as it is set to a non-existant get param.

https://github.com/civicrm/civicrm-core/pull/3588