Image data layer rng core dump

The back trace of the core dump file shows that the error occurs in `caffe::ImageDataLayer<float>::PrefetchRand`. The root cause is that `prefetch_rng_` is not initialized properly in `CreatePrefetchThread`. Luckily, there is no such bug in the DataLayer and the WindowDataLayer.
Is this change intended? I guess it is fine to initialize a RNG and not use it, but this modification seems unnecessary...