* Initial plan
* Fix crash when adjusting volume via button while audio device is disabled
When both input and output are disabled, UpdateDeviceState() sets dev_ to
nullptr. Pressing a volume button calls SetOutputVolume() which previously
called esp_codec_dev_set_out_vol(dev_, volume) without null-checking dev_,
causing a crash via ESP_ERROR_CHECK.
Fix: Add null guard for dev_ and mutex lock for thread safety. The volume
is still saved via AudioCodec::SetOutputVolume() and will be applied when
the device is reopened by UpdateDeviceState().
Agent-Logs-Url: https://github.com/78/xiaozhi-esp32/sessions/945c653a-ed16-49af-aefe-5cfb473402c6
Co-authored-by: 78 <4488133+78@users.noreply.github.com>
---------
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: 78 <4488133+78@users.noreply.github.com>
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