[FIX] mrp: prevent recursion from phantom boms with no lines

if the bom is phantom and has no line, we attempt to find a new bom with the default product uom
This is possible that we find the same bom that the current one
In such a case, we must not explode, to avoid recursion.
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32