Fix empty vocabulary issue.
Fix empty vocabulary issue.

Summary:
Training with small files and large -minCount values can lead to empty vocabulary.
In that case, fastText hangs indefinitely.

Reviewed By: piotr-bojanowski

Differential Revision: D3810851

fbshipit-source-id: 3be1a4da943c07845d00377f9d7a3d6b93f3ddd4

magicBus234/fastTextJob,facebookresearch/fastText,facebookresearch/fastText,magicBus234/fastTextJob,facebookresearch/fastText,magicBus234/fastTextJob,magicBus234/fastTextJob,facebookresearch/fastText,facebookresearch/fastText,facebookresearch/fastText