avcodec/h264_direct: Fix runtime error: signed integer overflow: 2147483647 - -14133 cannot be represented in type 'int'

Fixes: 755/clusterfuzz-testcase-5369072516595712

See: [FFmpeg-devel] [PATCH 1/2] avcodec/h264_direct: Fix runtime error: signed integer overflow: 2147483647 - -14133 cannot be represented in type 'int'

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

libavcodec/h264_direct.c

@@ -39,7 +39,12 @@ static int get_scale_factor(H264SliceContext *sl,
                             int poc, int poc1, int i)
 {
     int poc0 = sl->ref_list[0][i].poc;
-    int td = av_clip_int8(poc1 - poc0);
+    int64_t pocdiff = poc1 - (int64_t)poc0;
+    int td = av_clip_int8(pocdiff);
+
+    if (pocdiff != (int)pocdiff)
+        avpriv_request_sample(sl->h264->avctx, "pocdiff overflow\n");
+
     if (td == 0 || sl->ref_list[0][i].parent->long_ref) {
         return 256;
     } else {