Preloads fail silently when given incorrect query

[BrucePerens]

I gave a preload an incorrect query, in an ill-considered attempt to optimize:

preload_locations(LocationQuery.new.preload_url_path(URLPathQuery.new.limit(1))

The limit(1) is incorrect, preload_locations will retrieve more than one object. The result was an object with @_url_path_preloaded=true, and @_preloaded_url_path=nil, and no complaint from the preload code.