command: fix segfault with playlist-{next,prev}-playlist

This was wrongly assuming that playlist_path is always set for the
current playlist entry, but it's only set when a file was added by
expanding a playlist.

The crash in playlist_get_first_in_next_playlist can be reproduced with
mpv foo.mkv foo.zip, playlist-next, playlist-prev,
playlist-next-playlist. You need to run playlist-next, playlist-prev
first because foo.zip's playlist_path is NULL until you do that, which
makes playlist_get_first_in_next_playlist return immediately.

The crash in cmd_playlist_next_prev_playlist can be replicated with mpv
--loop-playlist foo.zip foo.mkv, running playlist-next until foo.mkv,
and playlist-play-next. Again, you need to open foo.zip first or its
playlist_path is NULL which skips running strcmp(entry->playlist_path,
mpctx->playlist->current->playlist_path).

Fixes https://github.com/mpv-player/mpv/issues/12495#issuecomment-1760968608