[-] BO : fixed bug loading wrong root category in AdminCategoriesController

Fix wrong root category being loaded.
There's an issue if you activate multishop and create a new root category which loads the wrong category and causes, at least, 'Add new Category' and 'Export' in AdminCategories to redirect back to the category list (e.g. impossible to add/remove categories).
Merge pull request #1121 from dlage/patch-1

[-] BO : fixed bug loading wrong root category in AdminCategoriesController