Fix heap-use-after-free

[dforsi]

This PR fixes issue #2059 but breaks the device pulldown because when changing the current device it doesn't add the new one.

AddressSanitizer: heap-use-after-free /home/sdrangel/sdrbase/audio/audiofifo.cpp:80 in AudioFifo::write(unsigned char const*, unsigned int) AddressSanitizer: heap-use-after-free /home/sdrangel/sdrbase/audio/audiofifo.h:49 in AudioFifo::fill() const

I tested the audio input with my radio and a websdr and the audio output only with the loudspeakers.