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/pcm_utils.h

@@ -51,6 +51,8 @@ void pcm_volume(char *buffer, int bufferSize,
 void pcm_mix(char *buffer1, const char *buffer2, size_t size,
              const struct audio_format *format, float portion1);
 
+void pcm_convert_init(struct pcm_convert_state *state);
+
 size_t pcm_convert(const struct audio_format *inFormat,
 		   const char *inBuffer, size_t inSize,
 		   const struct audio_format *outFormat,