Predict empty answer with high score

[Dabulv]

When predicting answer, sometimes I get empty answer, however the score is high enough. Finally I locate the problem. A function called _get2 at line 284 and 382 in file basic/evaluator.py, the judgement if len(xi[span[0][0]]) <= span[1][1]: should be if len(xi[span[0][0]]) < span[1][1]:. When the stop position is at the end of context, span[1][1] is equal with len(xi[span[0][0]]).