[opus] Bugfix for OPUS_GET_IN_DTX

[Gustaf Ullberg]

We (WebRTC) found a bug in the current implementation of
OPUS_GET_IN_DTX and I just created a pull request for a fix on the
Opus GitHub page. It would be great if maintainers could take a look.
https://github.com/xiph/opus/pull/156

Patch attached.

Best regards
Gustaf

>From the PR:
The current implementation of OPUS_GET_IN_DTX does not properly handle
the case when only the middle channel is encoded in Silk stereo.

This PR fixes the bug by only looking at noSpeechCouter for the second
channel (side channel) if the previous frame was not "only middle".

The bug was found by a failing WebRTC test case:
https://webrtc-review.googlesource.com/c/src/+/159709
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