MP4: do not set total tracks to 0

That doesn't really make sense

modules/demux/mp4/mp4.c

@@ -1025,9 +1025,12 @@ static int Control( demux_t *p_demux, int i_query, va_list args )
                     snprintf( psz_trck, sizeof( psz_trck ), "%i",
                               p_0xa9xxx->data.p_trkn->i_track_number );
                     vlc_meta_SetTrackNum( p_meta, psz_trck );
+                    if( p_0xa9xxx->data.p_trkn->i_track_total > 0 )
+                    {
                         snprintf( psz_trck, sizeof( psz_trck ), "%i",
                                   p_0xa9xxx->data.p_trkn->i_track_total );
                         vlc_meta_Set( p_meta, vlc_meta_TrackTotal, psz_trck );
+                    }
                     break;
                 }
                 case ATOM_0xa9cmt: /* Commment */