avcodec/ylc: Check count in build_vlc()

Fixes: runtime error: signed integer overflow: 211633430 + 2147483647 cannot be represented in type 'int'
Fixes: 1874/clusterfuzz-testcase-minimized-5037763613163520

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

libavcodec/ylc.c

@@ -109,7 +109,7 @@ static int build_vlc(AVCodecContext *avctx, VLC *vlc, const uint32_t *table)
             int new_node = j;
             int first_node = cur_node;
             int second_node = cur_node;
-            int nd, st;
+            unsigned nd, st;
 
             nodes[cur_node].count = -1;
 
@@ -133,6 +133,10 @@ static int build_vlc(AVCodecContext *avctx, VLC *vlc, const uint32_t *table)
             st = nodes[first_node].count;
             nodes[second_node].count = 0;
             nodes[first_node].count  = 0;
+            if (nd >= UINT32_MAX - st) {
+                av_log(avctx, AV_LOG_ERROR, "count overflow\n");
+                return AVERROR_INVALIDDATA;
+            }
             nodes[cur_node].count = nd + st;
             nodes[cur_node].sym = -1;
             nodes[cur_node].n0 = cur_node;