User account create fails on Contributions using D6 w/email registration module

The problem is the email registration module hooks the user registration process and changes the username before Civi has a chance to look it up. This causes the lookup to fail and, if any payment processing was due to occur, you get redirected to the homepage instead of the payment gateway.

A patch is attached to detect the module and, if present, perform the lookup on email address instead - which fixes the issue.