demux: avi: default to first track if none selected

avoids failing with delayed start tracks

modules/demux/avi/avi.c

@@ -1346,10 +1346,17 @@ static int Demux_UnSeekable( demux_t *p_demux )
     }
 
     if( !p_stream_master )
+    {
+        if( p_sys->i_track )
+        {
+            p_stream_master = p_sys->track[0];
+        }
+        else
         {
             msg_Warn( p_demux, "no more stream selected" );
             return( 0 );
         }
+    }
 
     p_sys->i_time = AVI_GetPTS( p_stream_master );