Use malloc(1) as cookie rather than malloc(0)

malloc(0) can legally be NULL (or not unique).

modules/stream_out/dummy.c

@@ -80,7 +80,7 @@ static void Close( vlc_object_t * p_this )
 static sout_stream_id_t *Add( sout_stream_t *p_stream, es_format_t *p_fmt )
 {
     VLC_UNUSED(p_stream); VLC_UNUSED(p_fmt);
-    return malloc( 0 );
+    return malloc( 1 );
 }
 
 static int Del( sout_stream_t *p_stream, sout_stream_id_t *id )