Predict empty answer with high score

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]])`.