Check ABV when creating a new stock type

Absence of this check enabled new stock types to be created with ABVs
that were invalid according to the rules for the stock type's
department.

quicktill/stocktype.py

@@ -226,6 +226,11 @@ def finish_select(self):
         # Confirmation box time...
         if st is None:
             if self.allownew:
+                # Re-check, including checking the ABV this time
+                problem = self.validate_fields()
+                if problem:
+                    ui.infopopup([problem], title="Error")
+                    return
                 ui.infopopup(
                     ["There's no existing stock type that matches the "
                      "details you've entered.  Press Cash/Enter to "