Hotfix made #1019

[ArkadiyD]

With parameter hs of model set to 0 there are no segmentation faults (Gensim 0.13.3, both Python 2. and 3. , proved with ddd). So, proposed hotfix is to turn off hs mode when model is upgraded.

For #1019

[tmylk]

The segfault only happens in doc2vec codepath so this restriction is too strict.

[gojomo]

Also if someone intends a hierarchical-softmax model, and indeed has negative=0 so no negative-sampling happens, just turning off hs, making the model completely non-functional, is just breaking things a different way.