avformat/segafilm: Set keyframe

Fixes: use of uninitialized value
Fixes: 70871/clusterfuzz-testcase-minimized-ffmpeg_dem_SEGAFILM_fuzzer-5883617752973312

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/segafilm.c

@@ -234,6 +234,7 @@ static int film_read_header(AVFormatContext *s)
             else if (film->audio_type != AV_CODEC_ID_NONE)
                 audio_frame_counter += (film->sample_table[i].sample_size /
                     (film->audio_channels * film->audio_bits / 8));
+            film->sample_table[i].keyframe = 1;
         } else {
             film->sample_table[i].stream = film->video_stream_index;
             film->sample_table[i].pts = AV_RB32(&scratch[8]) & 0x7FFFFFFF;