avcodec/h264_slice: Fix integer overflow in implicit_weight_table()

Fixes: signed integer overflow: 2 * 2132811760 cannot be represented in type 'int'
Fixes: 11156/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-6237685933408256

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_slice.c

@@ -678,7 +678,7 @@ static void implicit_weight_table(const H264Context *h, H264SliceContext *sl, in
             cur_poc = h->cur_pic_ptr->field_poc[h->picture_structure - 1];
         }
         if (sl->ref_count[0] == 1 && sl->ref_count[1] == 1 && !FRAME_MBAFF(h) &&
-            sl->ref_list[0][0].poc + (int64_t)sl->ref_list[1][0].poc == 2 * cur_poc) {
+            sl->ref_list[0][0].poc + (int64_t)sl->ref_list[1][0].poc == 2LL * cur_poc) {
             sl->pwt.use_weight        = 0;
             sl->pwt.use_weight_chroma = 0;
             return;