fix comparator for decreasing function of threshold skipping

[nlpng]

What this patch does to fix the issue.

Fix the comparator condition in apply_thresholds.cpp for threshold skipping.
For decreasing case like this (58,-76,-211, -1), where the ts0 is 58, ts1 is -76, and ts2 is -211

Link to any relevant issues or pull requests.

#247

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[iizukak]

@nlpng Oh, thanks.
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[tsawada]

Readability Approval

[iizukak]

/ready

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