[FIX] account: correct `price_subtotal_signed` computation
[FIX] account: correct `price_subtotal_signed` computation

When computing default values, `self.company_id` may not be set. Use
explictly `self.invoice_id.company_id` to compute the signed subtotal.
Also, only do it when there is an invoice.

mkieszek/odoo,dfang/odoo,optima-ict/odoo,storm-computers/odoo,stephen144/odoo,laslabs/odoo,microcom/odoo,ygol/odoo,tvtsoft/odoo8,stephen144/odoo,syci/OCB,hip-odoo/odoo,ygol/odoo,bplancher/odoo,laslabs/odoo,ygol/odoo,tvtsoft/odoo8,Elico-Corp/odoo_OCB,optima-ict/odoo,tvtsoft/odoo8,alhashash/odoo,sysadminmatmoz/OCB,laslabs/odoo,optima-ict/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,hip-odoo/odoo,stephen144/odoo,storm-computers/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,bplancher/odoo,stephen144/odoo,microcom/odoo,Elico-Corp/odoo_OCB,dfang/odoo,microcom/odoo,syci/OCB,bplancher/odoo,alhashash/odoo,hip-odoo/odoo,alhashash/odoo,storm-computers/odoo,Elico-Corp/odoo_OCB,microcom/odoo,hip-odoo/odoo,ygol/odoo,storm-computers/odoo,syci/OCB,optima-ict/odoo,alhashash/odoo,storm-computers/odoo,sysadminmatmoz/OCB,mkieszek/odoo,dfang/odoo,syci/OCB,dfang/odoo,syci/OCB,ygol/odoo,sysadminmatmoz/OCB,microcom/odoo,laslabs/odoo,stephen144/odoo,sysadminmatmoz/OCB,alhashash/odoo,syci/OCB,bplancher/odoo,microcom/odoo,stephen144/odoo,hip-odoo/odoo,mkieszek/odoo,storm-computers/odoo,bplancher/odoo,hip-odoo/odoo,alhashash/odoo,dfang/odoo,sysadminmatmoz/OCB,tvtsoft/odoo8,optima-ict/odoo,Elico-Corp/odoo_OCB,tvtsoft/odoo8,ygol/odoo,tvtsoft/odoo8,ygol/odoo,mkieszek/odoo,sysadminmatmoz/OCB,bplancher/odoo,mkieszek/odoo,optima-ict/odoo,mkieszek/odoo,sysadminmatmoz/OCB,dfang/odoo