avformat/mpegts: Check desc_len / get8() return code

Fixes out of array read
Fixes: signal_sigsegv_844d59_10_signal_sigsegv_a17bb7_366_mpegts_mpeg2video_mp2_dvbsub_topfield.rec

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mpegts: use a padded buffer in read_sl_header()

Fixes overread
Fixes: asan_heap-oob_84f75d_8_asan_heap-oob_a2a00a_341_mbc.ts

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>