avcodec/mpeg4videodec: Fix runtime error: signed integer overflow: 53098 * 40448 cannot be represented in type 'int'

Fixes: 2106/clusterfuzz-testcase-minimized-6136503639998464

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

libavcodec/mpeg4videodec.c

@@ -2340,7 +2340,7 @@ static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
     if (s->pict_type != AV_PICTURE_TYPE_B) {
         s->last_time_base = s->time_base;
         s->time_base     += time_incr;
-        s->time = s->time_base * s->avctx->framerate.num + time_increment;
+        s->time = s->time_base * (int64_t)s->avctx->framerate.num + time_increment;
         if (s->workaround_bugs & FF_BUG_UMP4) {
             if (s->time < s->last_non_b_time) {
                 /* header is not mpeg-4-compatible, broken encoder,
@@ -2352,7 +2352,7 @@ static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
         s->pp_time         = s->time - s->last_non_b_time;
         s->last_non_b_time = s->time;
     } else {
-        s->time    = (s->last_time_base + time_incr) * s->avctx->framerate.num + time_increment;
+        s->time    = (s->last_time_base + time_incr) * (int64_t)s->avctx->framerate.num + time_increment;
         s->pb_time = s->pp_time - (s->last_non_b_time - s->time);
         if (s->pp_time <= s->pb_time ||
             s->pp_time <= s->pp_time - s->pb_time ||