Fix compute exponent when value is int and factor Decimal

Some Numeric Function field return integer instead of Decimal, so the test to
convert to Decimal should be on the result of the factor multiplication instead
of the value.

issue5106
review16791003

tryton/gui/window/view_form/model/field.py

@@ -327,8 +327,8 @@ class FloatField(Field):
                 p = digits[1]
             else:
                 d = value * factor
-                if not isinstance(value, Decimal):
-                    d = Decimal(repr(value * factor))
+                if not isinstance(d, Decimal):
+                    d = Decimal(repr(d))
                 p = -int(d.as_tuple().exponent)
             return locale.format('%.*f', (p, value * factor), True)
         else: