sbgdec: prevent NULL pointer access

Reviewed-by: Josh de Kock <josh@itanimul.li>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit dbefbb61b7)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/sbgdec.c

@@ -927,7 +927,7 @@ static void expand_timestamps(void *log, struct sbg_script *s)
         }
     }
     if (s->start_ts == AV_NOPTS_VALUE)
-        s->start_ts = s->opt_start_at_first ? s->tseq[0].ts.t : now;
+        s->start_ts = (s->opt_start_at_first && s->tseq) ? s->tseq[0].ts.t : now;
     s->end_ts = s->opt_duration ? s->start_ts + s->opt_duration :
                 AV_NOPTS_VALUE; /* may be overridden later by -E option */
     cur_ts = now;