avformat/segafilm: Only add index entries when the stream exists

Fixes null pointer dereference
Fixes: cb02dfb163ac833c04cace3d7e35b160/signal_sigsegv_e55c49_6326_8f25619fc86a00b303c57b7778baf70a.cpk

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/segafilm.c

@@ -241,6 +241,7 @@ static int film_read_header(AVFormatContext *s)
             film->sample_table[i].pts = AV_RB32(&scratch[8]) & 0x7FFFFFFF;
             film->sample_table[i].keyframe = (scratch[8] & 0x80) ? 0 : 1;
             video_frame_counter++;
+            if (film->video_type)
                 av_add_index_entry(s->streams[film->video_stream_index],
                                    film->sample_table[i].sample_offset,
                                    film->sample_table[i].pts,