Round quantity before using it on invoice line or stock move

Precision can be lost also with only addition and substraction because of
float, so quantity must be rounded before used on invoice line or stock move.

issue4191
review13551002

purchase.py

@@ -1291,11 +1291,12 @@ class PurchaseLine(ModelSQL, ModelView):
             if old_invoice_line.id not in skip_ids:
                 quantity -= Uom.compute_qty(old_invoice_line.unit,
                         old_invoice_line.quantity, self.unit)
-        invoice_line.quantity = quantity
 
         rounding = self.unit.rounding if self.unit else 0
-        if invoice_line.quantity < rounding:
+        invoice_line.quantity = Uom.round(quantity, rounding)
+        if invoice_line.quantity <= 0:
             return []
+
         invoice_line.unit = self.unit
         invoice_line.product = self.product
         invoice_line.unit_price = self.unit_price
@@ -1349,8 +1350,11 @@ class PurchaseLine(ModelSQL, ModelView):
             if move not in skip:
                 quantity -= Uom.compute_qty(move.uom, move.quantity,
                     self.unit)
-        if quantity < self.unit.rounding:
+
+        quantity = Uom.round(quantity, self.unit.rounding)
+        if quantity <= 0:
             return
+
         if not self.purchase.party.supplier_location:
             self.raise_user_error('supplier_location_required', {
                     'purchase': self.purchase.rec_name,