0b35b4c917 originally introduced sd_filter
to make a more general subtitle filter infrastructure. But when doing
so, it directly sets ft->event_format to ass_track->event_format in the
struct. The lifetime of ass_track and the sd_filter are not equivalent
which makes it easy to trigger undefined behavior. Notably, commit
cda8f1613f introduced assobjects_destroy
which can destroy ass_track anytime during runtime which means that the
string in ft->event_format is actually freed and should never be used.
Remedy this by simply doing a proper strdup when the filter inits with
ft as the parent so we avoid this scenario altogether. Fixex #13525.
8ba6d8f7a9