MultipleOutputs: ensure input_audio_format is zero-initialised

-fixes SIGABRT when mpd is started from a previously paused state
-regression introduced by commit: f5a923b9d16e4c63942a033d1bdb2ab150aae342:
'OutputAll: convert to class, move instance to class Partition'

-input_audio_format was previously declared using the static modifier
ensuring it was zero-initialised by default
-the current default-initialised input_audio_format contains garbage at
runtime which allows the AudioFormat.IsDefined() 'fail fast' test in
MultipleOutputs::Update to pass and the SIGABRT follows in
AudioOutput::Open when passed the invalid input_audio_format struct
-switching AudioFormat.IsDefined() for AudioFormat.IsValid() is an
alternative workaround

src/output/MultipleOutputs.cxx

@@ -37,6 +37,7 @@
 
 MultipleOutputs::MultipleOutputs(MixerListener &_mixer_listener)
 	:mixer_listener(_mixer_listener),
+	 input_audio_format(AudioFormat::Undefined()),
 	 buffer(nullptr), pipe(nullptr),
 	 elapsed_time(-1)
 {