lavf/segment: fix crash when failing to open segment list

This happens because segment_end() returns an error, so seg_write_packet
never proceeds to segment_start(), and seg->avf->pb is never re-set,
so we crash with a null pb when av_write_trailer flushes the packet
queue.

This doesn't seem to be clearly recoverable, so I'm just failing more
gracefully.

Repro:
ffmpeg -i input.ts -f segment -c copy -segment_list /noaxx.m3u8 test-%05d.ts

(assuming you don't have write access to /)