avcodec/truemotion2: Move skip computation after checks

Fixes: runtime error: signed integer overflow: 630067357 * 4 cannot be represented in type 'int'
Fixes: 2233/clusterfuzz-testcase-minimized-5943031318446080

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

libavcodec/truemotion2.c

@@ -298,15 +298,15 @@ static int tm2_read_stream(TM2Context *ctx, const uint8_t *buf, int stream_id, i
     /* get stream length in dwords */
     bytestream2_init(&gb, buf, buf_size);
     len  = bytestream2_get_be32(&gb);
-    skip = len * 4 + 4;
 
     if (len == 0)
         return 4;
 
-    if (len >= INT_MAX / 4 - 1 || len < 0 || skip > buf_size) {
+    if (len >= INT_MAX / 4 - 1 || len < 0 || len * 4 + 4 > buf_size) {
         av_log(ctx->avctx, AV_LOG_ERROR, "Error, invalid stream size.\n");
         return AVERROR_INVALIDDATA;
     }
+    skip = len * 4 + 4;
 
     toks = bytestream2_get_be32(&gb);
     if (toks & 1) {