avcodec/h264idct_template: Fix integer overflow in ff_h264_chroma422_dc_dequant_idct()

Fixes: signed integer overflow: 241173056 + 1953511200 cannot be represented in type 'int'
Fixes: 26086/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5068366420901888

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d198362839)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264idct_template.c

@@ -278,7 +278,7 @@ void FUNCC(ff_h264_chroma422_dc_dequant_idct)(int16_t *_block, int qmul){
     const int stride= 16*2;
     const int xStride= 16;
     int i;
-    int temp[8];
+    unsigned temp[8];
     static const uint8_t x_offset[2]={0, 16};
     dctcoef *block = (dctcoef*)_block;