avcodec/h264idct_template: Fix integer overflow in ff_h264_chroma422_dc_dequant_idct()

Fixes: signed integer overflow: -2105540608 - 2105540608 cannot be represented in type 'int'
Fixes: 26870/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5656647567147008

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

libavcodec/h264idct_template.c

@@ -283,8 +283,8 @@ void FUNCC(ff_h264_chroma422_dc_dequant_idct)(int16_t *_block, int qmul){
     dctcoef *block = (dctcoef*)_block;
 
     for(i=0; i<4; i++){
-        temp[2*i+0] = block[stride*i + xStride*0] + block[stride*i + xStride*1];
-        temp[2*i+1] = block[stride*i + xStride*0] - block[stride*i + xStride*1];
+        temp[2*i+0] = block[stride*i + xStride*0] + (unsigned)block[stride*i + xStride*1];
+        temp[2*i+1] = block[stride*i + xStride*0] - (unsigned)block[stride*i + xStride*1];
     }
 
     for(i=0; i<2; i++){