Change parenthesis in menu_link_save() to express intended order of operations

This is a cosmetic change in followup of #1338282: Fix php notice in menu_link_save().
Current code
if (!$existing_item || (array_intersect_key($item, $existing_item)) != $existing_item)
has meaningless parenthesis around array_intersect_key().
This should be changed to
if (!$existing_item || (array_intersect_key($item, $existing_item) != $existing_item))
(suggested patch) or
if (!$existing_item || array_intersect_key($item, $existing_item) != $existing_item)
All three variants are syntactically equivalent, yet the second one suggests self-explaining order of operations