[FIX] stock, multicompany: avoid crashing when computing inventory levels and no warehouse/locations exists for a company

This is a cherry-pick backport of the trunk fix for bug 921561,
originally merged in trunk at revid fp@openerp.com-20130224101111-zz5n34c5fmr4n8jt

lp bug: https://launchpad.net/bugs/921561 fixed

bzr revid: odo@openerp.com-20130417132021-pmf0cdvwaxrsx9ew

addons/stock/product.py

@@ -235,6 +235,8 @@ class product_product(osv.osv):
         else:
             location_ids = []
             wids = warehouse_obj.search(cr, uid, [], context=context)
+            if not wids:
+                return res
             for w in warehouse_obj.browse(cr, uid, wids, context=context):
                 location_ids.append(w.lot_stock_id.id)