BUG: Prevent crash in vwap transform due to non-existent member.
BUG: Prevent crash in vwap transform due to non-existent member.

The WrongDataForTransform was referencing a `self.fields` member,
which did not exist.

Add a self.fields member set to `price` and `volume` and use
it to iterate over during the check.

dmitriz/zipline,dmitriz/zipline,euri10/zipline,morrisonwudi/zipline,sketchytechky/zipline,stkubr/zipline,cmorgan/zipline,ChinaQuants/zipline,jimgoo/zipline-fork,jordancheah/zipline,euri10/zipline,nborggren/zipline,ronalcc/zipline,wilsonkichoi/zipline,otmaneJai/Zipline,mattcaldwell/zipline,iamkingmaker/zipline,dkushner/zipline,stkubr/zipline,AlirezaShahabi/zipline,enigmampc/catalyst,DVegaCapital/zipline,jordancheah/zipline,keir-rex/zipline,florentchandelier/zipline,dkushner/zipline,YuepengGuo/zipline,humdings/zipline,StratsOn/zipline,MonoCloud/zipline,cmorgan/zipline,bartosh/zipline,kmather73/zipline,erikness/AlephOne,ChinaQuants/zipline,zhoulingjun/zipline,enigmampc/catalyst,erikness/AlephOne,chrjxj/zipline,mattcaldwell/zipline,quantopian/zipline,humdings/zipline,keir-rex/zipline,umuzungu/zipline,aajtodd/zipline,CDSFinance/zipline,michaeljohnbennett/zipline,alphaBenj/zipline,CDSFinance/zipline,aajtodd/zipline,dhruvparamhans/zipline,joequant/zipline,sketchytechky/zipline,ronalcc/zipline,dhruvparamhans/zipline,MonoCloud/zipline,grundgruen/zipline,kmather73/zipline,iamkingmaker/zipline,jimgoo/zipline-fork,semio/zipline,magne-max/zipline-ja,semio/zipline,alphaBenj/zipline,StratsOn/zipline,YuepengGuo/zipline,morrisonwudi/zipline,magne-max/zipline-ja,zhoulingjun/zipline,wilsonkichoi/zipline,AlirezaShahabi/zipline,joequant/zipline,florentchandelier/zipline,chrjxj/zipline,umuzungu/zipline,DVegaCapital/zipline,wubr2000/zipline,wubr2000/zipline,otmaneJai/Zipline,Scapogo/zipline,bartosh/zipline,Scapogo/zipline,michaeljohnbennett/zipline,nborggren/zipline,quantopian/zipline,gwulfs/zipline,gwulfs/zipline,grundgruen/zipline