alsa-mixer: check if mapping is NULL before using it
Fix Debian bullseye bug where adding this line makes pulseaudio crash on startup
load-module module-alsa-sink device=hw:1,0 control=Wave
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=989103
Fixes: dacfcbb0 ("alsa-ucm: use the proper mixer name for ucm pcm sink/source")
Since this is a regression, adding to 15.0
milestone