ChoiceIterator: skip symbol without choices

[bertsky]

This fixes a segfault when running with lstm_choice_mode=2 and querying the choice iterator.

[stweil]

Yes, this will fix the crash caused by an empty first alternative. This would also fix it:

 if (word_res_->CTC_symbol_choices[0].empty() ||
     strcmp(word_res_->CTC_symbol_choices[0][0].first, " ")) {

Are you sure that your fix is the right one? And I wonder whether symbols without a choice may exist at all or whether there is a bug somewhere else.

[bertsky]

This would also fix it:

 if (word_res_->CTC_symbol_choices[0].empty() ||
     strcmp(word_res_->CTC_symbol_choices[0][0].first, " ")) {

Indeed it would. It's not the same thing (blanks_before_word_ from the normal iterator would not always be skipped), and I have not tested your variant.

Are you sure that your fix is the right one? And I wonder whether symbols without a choice may exist at all or whether there is a bug somewhere else.

Probably just a follow-up, yes.