Fix a bug in mean calculation of 'ys_probs'

[aask1357]

A vector variable best_hyp.ys_probs contains the probabilities of each token in a matched keyword whose length is level.
The code checks if the mean of best_hyp.ys_probs is greater than or equal to a threshold.

Problem

Currently, when calculating the mean, the index ranges from 1 to level.
This excludes the first token (at index 0) and includes an undefined value at the end.
In most cases, this undefined value is zero so the code seems to work fine.

Solution

Change the index to range from 0 to level-1.

[csukuangfj]

@pkufool Could you have a look?

[pkufool]

@aask1357 Though I can't recall why I wrote down this code before，after reviewing the current code carefully, I think you are right, Thanks!