[SPARK-7663] [MLLIB] Add requirement for word2vec model

JIRA issue [link](https://issues.apache.org/jira/browse/SPARK-7663).

We should check the model size of word2vec, to prevent the unexpected empty.

CC srowen.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #6228 from yinxusen/SPARK-7663 and squashes the following commits:

21770c5 [Xusen Yin] check the vocab size
54ae63e [Xusen Yin] add requirement for word2vec model