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>

libavformat/mpegts.c

@@ -1994,7 +1994,7 @@ static void sdt_cb(MpegTSFilter *filter, const uint8_t *section, int section_len
                 break;
             desc_len = get8(&p, desc_list_end);
             desc_end = p + desc_len;
-            if (desc_end > desc_list_end)
+            if (desc_len < 0 || desc_end > desc_list_end)
                 break;
 
             av_dlog(ts->stream, "tag: 0x%02x len=%d\n",