demux: constify a struct member

Fixes a segfault when an invalid demuxer is given due to
uninitialized use of `filename` after goto.

demux/demux.c

@@ -3263,7 +3263,7 @@ struct parent_stream_info {
     bool is_streaming;
     int stream_origin;
     struct mp_cancel *cancel;
-    char *filename;
+    const char *filename;
 };
 
 static struct demuxer *open_given_type(struct mpv_global *global,
@@ -3433,7 +3433,7 @@ static struct demuxer *demux_open(struct stream *stream,
         .is_streaming = stream->streaming,
         .stream_origin = stream->stream_origin,
         .cancel = cancel,
-        .filename = talloc_strdup(NULL, stream->url),
+        .filename = stream->url,
     };
 
     // Test demuxers from first to last, one pass for each check_levels[] entry
@@ -3455,7 +3455,6 @@ static struct demuxer *demux_open(struct stream *stream,
     }
 
 done:
-    talloc_free(sinfo.filename);
     talloc_free(log);
     return demuxer;
 }