transcode: reduce decoder aspect ratio fraction

avoids integer overflow with aspect ratio = BIG:BIG
fixes #4312

modules/stream_out/transcode/video.c

@@ -495,6 +495,12 @@ static void transcode_video_encoder_init( sout_stream_t *p_stream,
     if( id->p_encoder->fmt_out.video.i_sar_num <= 0 ||
         id->p_encoder->fmt_out.video.i_sar_den <= 0 )
     {
+        vlc_ureduce( &id->p_decoder->fmt_out.video.i_sar_num,
+                     &id->p_decoder->fmt_out.video.i_sar_den,
+                     id->p_decoder->fmt_out.video.i_sar_num,
+                     id->p_decoder->fmt_out.video.i_sar_den,
+                     0 );
+
         id->p_encoder->fmt_out.video.i_sar_num = id->p_decoder->fmt_out.video.i_sar_num * i_src_width / i_dst_width;
         id->p_encoder->fmt_out.video.i_sar_den = id->p_decoder->fmt_out.video.i_sar_den * i_src_height / i_dst_height;
     }