[FIX] account: invoice report, advances counted twice

The below revision force the use of `ABS` on the
`price_subtotal_signed` because of a migration issue:
649b289

This leads to issues regarding the advances, in
`out_invoice` invoices, where this amount
`price_subtotal_signed` is negative, and should
remain negative in this report, otherwise
the advance is counted twice: Once with
the advance invoice, and a second in the actual
invoice where the advance is removed with a line
with a negative `price_subtotal`

e.g.
 - A sale order of 500€ without taxes
 - Create an advance invoice for an advance payment of 10%: 50€ without taxes
 - A second invoice with the remaining: 450€ without taxes

In the invoices report, the total without taxes for this customer was
displayed as 600€ (50 + 500 + 50)
instead of   500€ (50 + 500 - 50)

opw-682390

addons/account/report/account_invoice_report.py

@@ -115,11 +115,17 @@ def _sub_select(self):
                             THEN (- ail.quantity) / u.factor * u2.factor
                             ELSE ail.quantity / u.factor * u2.factor
                         END) AS product_qty,
-                    SUM(ABS(ail.price_subtotal_signed) * CASE
-                        WHEN ai.type::text = ANY (ARRAY['out_refund'::character varying::text, 'in_invoice'::character varying::text])
-                            THEN -1
-                            ELSE 1
-                        END
+                    SUM(ABS(ail.price_subtotal_signed)
+                        * CASE
+                            WHEN ail.price_subtotal < 0
+                                THEN -1
+                                ELSE 1
+                            END
+                        * CASE
+                            WHEN ai.type::text = ANY (ARRAY['out_refund'::character varying::text, 'in_invoice'::character varying::text])
+                                THEN -1
+                                ELSE 1
+                            END
                     ) AS price_total,
                     SUM(ABS(ail.price_subtotal_signed)) / CASE
                         WHEN SUM(ail.quantity / u.factor * u2.factor) <> 0::numeric