Description tag fix

Fixes description tag writing in a matroska container (bug tracker #1689). This is the simplest way to fix the issue.

Observations:
* const dvr_entry_t *de = NULL (always and so it's useless)
 * muxer_tvh.c:178: mk_mux_write_meta(tm->tm_ref, NULL, eb)
* Why those description/summary string pointers are stored in multiple places?
 * struct epg_episode, struct epg_broadcast, struct dvr_entry