pcm_utils: added pcm_convert_init()

Instead of manually calling memset(0) on the pcm_convert_state struct,
client code should use a library function from pcm_utils.c.  This way,
we can change the semantics of the struct easily.

src/decoder_api.c

@@ -47,7 +47,7 @@ void decoder_initialized(struct decoder * decoder,
 {
 	assert(dc.state == DECODE_STATE_START);
 
-	memset(&decoder->conv_state, 0, sizeof(decoder->conv_state));
+	pcm_convert_init(&decoder->conv_state);
 
 	if (audio_format != NULL) {
 		dc.audioFormat = *audio_format;