doc_topic table not written after training with -out_of_core option

[stsk129]

Hi @feiga thanks for the implementation! I noticed an issue when training the model with multiple data blocks and -out_of_core computing on a single machine. Training finishes successfully but the program doesn't terminate. The doc_topic.0 file is created but not written. The other doc_topic files are not created.
The program works fine with -out_of_core disabled. I tested it on Ubuntu 16.04.
Bests,
Stan