[MAD Input] Initialize state variable, fix crash

This should prevent a crash if the input is recycled for another file,
which would cause the output buffer to be freed, but the output size to
contain a count on first call to readAudio, which would cause a memory
access crash.

Possibly fixes #269

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Plugins/MAD/MADDecoder.m

@@ -387,6 +387,7 @@
 	mad_synth_init(&_synth);
 
 	_firstFrame = YES;
+	_outputFrames = 0;
 	// DLog(@"OPEN: %i", _firstFrame);
 
 	inputEOF = NO;