fix for invalid __channelCount in ~AudioBufferManager()

[lyusupov]

AudioOutputPWM method from ESP8266Audio library plays very first audio file just fine,
but gets stuck on a second one.

This is the gdb backtrace:

(gdb) bt
#0  dma_channel_get_irq0_status (channel=3)
    at /home/user/Arduino/hardware/pico/rp2040//pico-sdk/src/rp2_common/hardware_dma/include/hardware/dma.h:633
#1  AudioBufferManager::_irq ()
    at /home/user/Arduino/hardware/pico/rp2040/libraries/AudioBufferManager/src/AudioBufferManager.cpp:281
#2  0x20000cb8 in irq_handler_chain_slots ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb)

It was found out that the issue is caused by ~AudioBufferManager() destructor that have not freed the irq resource properly after the first run.

This commit fixes the issue.
AudioOutputPWM (ESP8266Audio) can play multiple of WAV files now.

[earlephilhower]

Thanks, appreciate the PR! Looks good and logically makes sense.