avcodec/h264_direct: Fix runtime error: signed integer overflow: -9 - 2147483647 cannot be represented in type 'int'

Fixes: 864/clusterfuzz-testcase-4774385942528000

See: [FFmpeg-devel] [PATCH 1/2] avcodec/h264_direct: Fix runtime error: signed integer overflow: 2147483647 - -14133 cannot be represented in type 'int'
See: [FFmpeg-devel] [PATCH 2/2] avcodec/h264_direct: Fix runtime error: signed integer overflow: -9 - 2147483647 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

@@ -48,8 +48,13 @@ static int get_scale_factor(H264SliceContext *sl,
     if (td == 0 || sl->ref_list[0][i].parent->long_ref) {
         return 256;
     } else {
-        int tb = av_clip_int8(poc - poc0);
+        int64_t pocdiff0 = poc - (int64_t)poc0;
+        int tb = av_clip_int8(pocdiff0);
         int tx = (16384 + (FFABS(td) >> 1)) / td;
+
+        if (pocdiff0 != (int)pocdiff0)
+            av_log(sl->h264->avctx, AV_LOG_DEBUG, "pocdiff0 overflow\n");
+
         return av_clip_intp2((tb * tx + 32) >> 6, 10);
     }
 }