[FIX] crm: check if name is present in measures

Fix an error which occurs during an attempt to access the string
attribute of a non existing measure
If a pivot view is defined with a "string" attribute of a field of
type "row", e.g.

   <field name="stage_id" string="Stage" type="row"/>

an error was raised "TypeError: measures[name] is undefined"

closes odoo/odoo#42941

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>