Output: start with a null mixer.

There are code paths (mostly error cases) in which it is possible to
initialize an AudioOutput and then kill it without ever calling
audio_output_new().  In such a case, its destructor will attempt to
free a mixer that was never initialized, leading to an attempt to
take out a lock on a mutex that was similarly never initialized,
which hangs forever.

Fix by always initializing the mixer appropriately.

src/output/Init.cxx

@@ -48,6 +48,7 @@
 
 AudioOutput::AudioOutput(const AudioOutputPlugin &_plugin)
 	:plugin(_plugin),
+	 mixer(nullptr),
 	 enabled(true), really_enabled(false),
 	 open(false),
 	 pause(false),