[MERGE] [FIX] account_voucher: in recompute_voucher_lines, ensure the price value...

Using the remaining amount for writeoff does not make sense as the amount is already in credit or debit lines.
This issue could be observed in the payment wizard where the execution of _compute_writeoff_amount (through onchange) would be a negative value at the first execution.

bzr revid: mat@openerp.com-20140423101847-s0efihzqe7jcdcps