pcm_convert: return PCM buffer from pcm_convert()

Removed yet another superfluous buffer layer: return the PCM buffer
from pcm_convert() instead of copying PCM data into the
caller-supplied buffer.

src/output_init.c

@@ -90,9 +90,6 @@ int audio_output_init(struct audio_output *ao, ConfigParam * param)
 	ao->open = false;
 	ao->reopen_after = 0;
 
-	ao->convBuffer = NULL;
-	ao->convBufferLen = 0;
-
 	pcm_convert_init(&ao->convState);
 
 	if (format) {