Fixed crashing in LDA's learn_batch function in case l.sorted_features is empty. (When a socket connection is dropped, l.sorted_features may end up being empty. In that case, l.sorted_features[0] does not exist and therefore its address cannot be taken. This is probably not the best place nor the best way to fix it, though - but given my limited understanding of VW, I felt this was the best I could do, for now at least.)