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.
Yes, it is. The change fixes the segementation fault caused by uninitialized `prefetch_rng_`. This is indeed a bug considering where `prefetch_rng_` is used in the prefetch thread. Perhaps it was not caught earlier because the ImageDataLayer parameter combinations did not run through the specific code path.

The DataLayer and the WindowDataLayer are both correct in this respect.