avcodec/mpeg12enc: Use av_rescale() in vbv_buffer_size computation

Fixes: signed integer overflow: 20 * 2314885530818453759 cannot be represented in type 'long'
Fixes: 69098/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG2VIDEO_fuzzer-6107989688778752

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

libavcodec/mpeg12enc.c

@@ -330,7 +330,7 @@ static void mpeg1_encode_sequence_header(MpegEncContext *s)
     else
         /* VBV calculation: Scaled so that a VCD has the proper
          * VBV size of 40 kilobytes */
-        vbv_buffer_size = ((20 * s->bit_rate) / (1151929 / 2)) * 8 * 1024;
+        vbv_buffer_size = av_rescale_rnd(s->bit_rate, 20, 1151929 / 2, AV_ROUND_ZERO) * 8 * 1024;
     vbv_buffer_size = (vbv_buffer_size + 16383) / 16384;
 
     put_sbits(&s->pb, 18, v);