[FIX] product: crash on product search

- Go to Sales > Product Variants
- In the search field, type a letter (e.g. 'p'), then select a
  corresponding pricelist ('Public Pricelist' in demo data)

=> Crash

This is because `name_search` returns a list of tuple, not a browse
record.

Fixes #15443

addons/product/models/product.py

@@ -177,7 +177,9 @@ class ProductProduct(models.Model):
 
             # Support context pricelists specified as display_name or ID for compatibility
             if isinstance(pricelist_id_or_name, basestring):
-                pricelist = self.env['product.pricelist'].name_search(pricelist_id_or_name, operator='=', limit=1)
+                pricelist_name_search = self.env['product.pricelist'].name_search(pricelist_id_or_name, operator='=', limit=1)
+                if pricelist_name_search:
+                    pricelist = self.env['product.pricelist'].browse([pricelist_name_search[0][0]])
             elif isinstance(pricelist_id_or_name, (int, long)):
                 pricelist = self.env['product.pricelist'].browse(pricelist_id_or_name)