EBML_STOP leaves matroska->current_id set. Then matroska_read_seek changes
the stream position without resetting current_id.  The next
matroska_parse_cluster  fails due to calculation of incorrect pos.  So clear
current_id when avio_seek happens in matroska_read_seek.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>