avformat/segment: Fix segfault on allocation error, avoid allocation

avformat/segment: Fix segfault on allocation error, avoid allocation

If the user has set none of the options specifying the segments'
durations, a default value of 2s is used by duplicating a "2" string and
using av_parse_time() on it. Yet duplicating the string was unchecked
and if the allocation failed, one would get a segfault in
av_parse_time().

This commit solves this by turning said option into an option of type
AV_OPT_TYPE_DURATION (which also uses av_parse_time() internally),
avoiding duplicating the string altogether.

Reviewed-by: Ridley Combs <rcombs@rcombs.me>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>