media_track: fix tracklist leak when no track found

When count is 0 the list is still allocated. Either we should not allocate
one, we free it locally, or we pass it on as it.
The latter is similar to the code from 40c14fbee4
in libvlc_media_tracklist_from_es_array().

Introduced in 057f3ef9e8.

lib/media_track.c

@@ -270,7 +270,7 @@ libvlc_media_tracklist_from_player( vlc_player_t *player,
     libvlc_media_tracklist_t *list = libvlc_media_tracklist_alloc( count );
 
     if( count == 0 || list == NULL )
-        return NULL;
+        return list;
 
     for( size_t i = 0; i < count; ++i )
     {