fix data checking report bug (#403)

Summary:
The original code reports the size of a valid sample instead of an invalid one when raising an Exception , which will make people confused.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/403

Differential Revision: D13391431

Pulled By: myleott

fbshipit-source-id: 4642ed027c0f664424fc5a9baf4363791144feaf