TargetImagesController#factory_keys should guard against params[:target_image] being nil

[n1zyy]

This is probably an edge case (it was only exposed by a separate bug (not in Tim) causing data to be posted to the wrong URL), but the factory_keys method in TargetImagesController begins by checking params[:target_image][:percent_complete].

params[:target_image] can be nil, at least in the case of bad data being passed in, causing the method to raise an undefined method[]' for nil:NilClass` exception.