avcodec/h264_cavlc: Check mb_skip_run

Fixes: 10300/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-6292205497483264
Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'

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

libavcodec/h264_cavlc.c

@@ -714,8 +714,14 @@ int ff_h264_decode_mb_cavlc(const H264Context *h, H264SliceContext *sl)
     cbp = 0; /* avoid warning. FIXME: find a solution without slowing
                 down the code */
     if (sl->slice_type_nos != AV_PICTURE_TYPE_I) {
-        if (sl->mb_skip_run == -1)
-            sl->mb_skip_run = get_ue_golomb_long(&sl->gb);
+        if (sl->mb_skip_run == -1) {
+            unsigned mb_skip_run = get_ue_golomb_long(&sl->gb);
+            if (mb_skip_run > h->mb_num) {
+                av_log(h->avctx, AV_LOG_ERROR, "mb_skip_run %d is invalid\n", mb_skip_run);
+                return AVERROR_INVALIDDATA;
+            }
+            sl->mb_skip_run = mb_skip_run;
+        }
 
         if (sl->mb_skip_run--) {
             if (FRAME_MBAFF(h) && (sl->mb_y & 1) == 0) {