[Warehouse] Prevent negative stock.

Task: https://accounts.openerp.com/web#id=10130&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.KCBJWsVnxINg746v
This is wrong.  You only need move.location_id.usage=='internal'.  qty and qty_available have nothing to do with it.  If you look at the _quant_create in stock/stock.py: you want to avoid the case where it creates a negative quant.