avcodec/mpeg12dec: Fixes runtime error: division by zero

Fixes: 1464/clusterfuzz-testcase-minimized-4925445571084288

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c0ece1f4ad)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg12dec.c

@@ -1291,7 +1291,8 @@ static int mpeg_decode_postinit(AVCodecContext *avctx)
 
     if (avctx->codec_id == AV_CODEC_ID_MPEG1VIDEO) {
         // MPEG-1 aspect
-        avctx->sample_aspect_ratio = av_d2q(1.0 / ff_mpeg1_aspect[s->aspect_ratio_info], 255);
+        AVRational aspect_inv = av_d2q(ff_mpeg1_aspect[s->aspect_ratio_info], 255);
+        avctx->sample_aspect_ratio = (AVRational) { aspect_inv.den, aspect_inv.num };
     } else { // MPEG-2
         // MPEG-2 aspect
         if (s->aspect_ratio_info > 1) {