Prevent last thread from exiting early on multiple iterations.

https://github.com/tmikolov/word2vec/issues/47
https://github.com/tmikolov/word2vec/pull/39
--- word2vec.c.orig
+++ word2vec.c
@@ -424,6 +424,7 @@ void *TrainModelThread(void *id) {
       last_word_count = 0;
       sentence_length = 0;
       fseek(fi, file_size / (long long)num_threads * (long long)id, SEEK_SET);
+      eof = 0;
       continue;
     }
     word = sen[sentence_position];