#11 undo Invoice on first invoice prevents further invoicing

[urugamai]

It appears that if you select the undo invoice button on the first invoice you create then you cannot create any more invoices. Possibly it would create an invoice after you waited for an entire billing period to pass. Fix is to set lastbilldate on the relevant entry in the projects table to a date before the required billing period (NULL did not work). Looked briefly at code but nothing jumped out at me as a quick fix for setting the $enddate value.