dashdec: Fix segfault on decoding segment timeline

If first_seq_no is not within the bounds of timelines then a segfault
will occur.
This patch removes the use of first_seq_no within the timelines array
It also adds first_seq_no to the value returned by calc_next_seg_no_from_timelines
(which allows for different values of 'startNumber')

Signed-off-by: Brendan McGrath <redmcg@redmandi.dyndns.org>