Since 062104d16e, we started saving cached
packets for subtitles. However, these can point to the same address as
what is stored in sub->new_segment. When a segment is updated, the
packet is potentially freed. Later during decoding, this can lead to a
double free since the cached packets will naturally try to free itself
and update. Fix this by simply always making sub->new_segment a full
copy of the packet so its lifetime doesn't have to be tied to the cached
packet stuff.