[FIX] account: prevent the residual amount of invoices/credit notes to be negative

bzr revid: qdp-launchpad@openerp.com-20130315130147-1n6g5z7nghmucibo

addons/account/account_invoice.py

@@ -99,6 +99,8 @@ class account_invoice(osv.osv):
                 for m in invoice.move_id.line_id:
                     if m.account_id.type in ('receivable','payable'):
                         result[invoice.id] += m.amount_residual_currency
+            #prevent the residual amount on the invoice to be less than 0
+            result[invoice.id] = max(result[invoice.id], 0.0)            
         return result
 
     # Give Journal Items related to the payment reconciled to this invoice