For a live mpd, if availabilityStartTime is missing, adaptive demux asserts
with: Unexpected critical/warning: gst_date_time_to_g_date_time: assertion
'datetime != NULL' failed.
This patch improves the error message to:
Unexpected critical/warning: gst_mpd_client_seek_to_time: assertion
'client->mpd_node->availabilityStartTime != NULL' failed
https://bugzilla.gnome.org/show_bug.cgi?id=757602