accounting: always compare rounded values when indexing balance transactions

To prevent rounding errors, we always compare rounded values to the
precision of the accounting currency. There was a place here where we
were using -= without rounding, which caused to detect a difference
between new and current stock and insert a line for 0 in an existing
instance for which some accounting lines were created with too precise
values - but not in a way that was detected by the assertions in
AccountingPeriod_createBalanceTransaction.

Rounding here as well solved the problem with that data.

bt5/erp5_accounting/DocumentTemplateItem/portal_components/document.erp5.BalanceTransaction.py

@@ -355,7 +355,9 @@ class BalanceTransaction(AccountingTransaction, Inventory):
 
         # matching_diff are negated later
         if matching_diff:
-          matching_diff['quantity'] -= round(new_stock['quantity'], precision)
+          matching_diff['quantity'] = round(
+            matching_diff['quantity'] - new_stock['quantity'],
+            precision)
           # Matching_diff and new_stock must be consistent.
           # both with total price or none.
           if matching_diff['total_price'] and new_stock['total_price']: