[transcode] Fix problem for file types where ffmpeg decode returns 0

It seems ffmpeg/libav reads a file type like Monkey's audio in large
chunks, so that each packet must be passed to the decoder multiple
times. The decoder will return 0 until the packet is completed. This
change makes forked-daapd able to deal with a return value of 0.

src/transcode.h

@@ -62,7 +62,7 @@ transcode_decoded_free(struct decoded_frame *decoded);
  * @out decoded   A newly allocated struct with a pointer to the frame and the
  *                stream. Must be freed with transcode_decoded_free().
  * @in  ctx       Decode context
- * @return        Bytes read if OK, negative if error, 0 if EOF
+ * @return        Positive if OK, negative if error, 0 if EOF
  */
 int
 transcode_decode(struct decoded_frame **decoded, struct decode_ctx *ctx);