[xcode] Fix for "Invalid return value 0 for stream protocol" log msg

Custom avio must not return 0, see https://patchwork.ffmpeg.org/project/ffmpeg/patch/20171025082258.32530-2-george@nsup.org/

src/avio_evbuffer.c

@@ -48,7 +48,8 @@ avio_evbuffer_read(void *opaque, uint8_t *buf, int size)
 
   ret = evbuffer_remove(ae->evbuf, buf, size);
 
-  return ret;
+  // Must return AVERROR, see avio.h: avio_alloc_context()
+  return (ret > 0) ? ret : AVERROR_EOF;
 }
 
 static int