fix permission check for polls for deleted proposals
fix permission check for polls for deleted proposals

If a proposal is deleted, the related selections and polls
are deleted. poll.selection will be None in this case.

phihag/adhocracy,alkadis/vcv,liqd/adhocracy,alkadis/vcv,liqd/adhocracy,SysTheron/adhocracy,phihag/adhocracy,SysTheron/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,phihag/adhocracy,alkadis/vcv,SysTheron/adhocracy,phihag/adhocracy,phihag/adhocracy,liqd/adhocracy,alkadis/vcv,liqd/adhocracy