Fix crash on bulk update and delete returned error.
Fix crash on bulk update and delete returned error.

gabrielrussell/mgo,hexadecy/mgo,DroiTaipei/mgo,Yottaa/mgo.v2,gabrielrussell/mgo,deafgoat/mgo,go-mgo/travis-test,hexadecy/mgo,go-mgo/travis-test,DroiTaipei/mgo,gabrielrussell/mgo,deafgoat/mgo,deafgoat/mgo,go-mgo/travis-test,jackspirou/mgo,go-mgo/travis-test,DroiTaipei/mgo,jackspirou/mgo,Yottaa/mgo.v2,jackspirou/mgo,deafgoat/mgo,hexadecy/mgo,hexadecy/mgo,DroiTaipei/mgo,Yottaa/mgo.v2,Yottaa/mgo.v2,gabrielrussell/mgo,jackspirou/mgo