avcodec/vp9_superframe_split_bsf: Fix integer overflow in frame_size/total_size checks

Fixes: signed integer overflow: -1698586465 + -551542752 cannot be represented in type 'int'
Fixes: 4490/clusterfuzz-testcase-minimized-5210014592532480

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

libavcodec/vp9_superframe_split_bsf.c

@@ -59,7 +59,7 @@ static int vp9_superframe_split_filter(AVBSFContext *ctx, AVPacket *out)
 
             if (in->size >= idx_size && in->data[in->size - idx_size] == marker) {
                 GetByteContext bc;
-                int total_size = 0;
+                int64_t total_size = 0;
 
                 bytestream2_init(&bc, in->data + in->size + 1 - idx_size,
                                  nb_frames * length_size);
@@ -70,7 +70,7 @@ static int vp9_superframe_split_filter(AVBSFContext *ctx, AVPacket *out)
                         frame_size |= bytestream2_get_byte(&bc) << (j * 8);
 
                     total_size += frame_size;
-                    if (total_size > in->size - idx_size) {
+                    if (frame_size < 0 || total_size > in->size - idx_size) {
                         av_log(ctx, AV_LOG_ERROR,
                                "Invalid frame size in a superframe: %d\n", frame_size);
                         ret = AVERROR(EINVAL);