demux: acquire lock before calling update_bytes_read

in->byte_level_seeks field is written and modified inside
update_bytes_read at the same time when demux_get_reader_state
is executing, which locks the demux thread mutex.  This results
in a data race, reported by Thread Sanitizer when playing mp3 file
of sufficient long length.

demux/demux.c

@@ -4137,10 +4137,10 @@ static void update_cache(struct demux_internal *in)
         stream_control(stream, STREAM_CTRL_GET_METADATA, &stream_metadata);
     }
 
-    update_bytes_read(in);
-
     pthread_mutex_lock(&in->lock);
 
+    update_bytes_read(in);
+
     if (do_update)
         in->stream_size = stream_size;
     if (stream_metadata) {