avcodec/dnxhddec: Check dc vlc

Fixes: signed integer overflow: 1024 + 2147483640 cannot be represented in type 'int'
Fixes: 4671/clusterfuzz-testcase-minimized-6027464343027712

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b2be76c0a4)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dnxhddec.c

@@ -377,6 +377,10 @@ static av_always_inline int dnxhd_decode_dct_block(const DNXHDContext *ctx,
 
     UPDATE_CACHE(bs, &row->gb);
     GET_VLC(len, bs, &row->gb, ctx->dc_vlc.table, DNXHD_DC_VLC_BITS, 1);
+    if (len < 0) {
+        ret = len;
+        goto error;
+    }
     if (len) {
         level = GET_CACHE(bs, &row->gb);
         LAST_SKIP_BITS(bs, &row->gb, len);
@@ -430,7 +434,7 @@ static av_always_inline int dnxhd_decode_dct_block(const DNXHDContext *ctx,
         GET_VLC(index1, bs, &row->gb, ctx->ac_vlc.table,
                 DNXHD_VLC_BITS, 2);
     }
-
+error:
     CLOSE_READER(bs, &row->gb);
     return ret;
 }