[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

mllib/src/main/scala/org/apache/spark/mllib/feature/Word2Vec.scala

@@ -158,6 +158,9 @@ class Word2Vec extends Serializable with Logging {
       .sortWith((a, b) => a.cn > b.cn)
     
     vocabSize = vocab.length
+    require(vocabSize > 0, "The vocabulary size should be > 0. You may need to check " +
+      "the setting of minCount, which could be large enough to remove all your words in sentences.")
+
     var a = 0
     while (a < vocabSize) {
       vocabHash += vocab(a).word -> a