avcodec/h264idct_template: Fix integer overflows in ff_h264_idct8_add()

Fixes: runtime error: signed integer overflow: 924846844 + 1457520640 cannot be represented in type 'int'
Fixes: 3416/clusterfuzz-testcase-minimized-6125587682820096

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

libavcodec/h264idct_template.c

@@ -107,10 +107,10 @@ void FUNCC(ff_h264_idct8_add)(uint8_t *_dst, int16_t *_block, int stride){
     }
     for( i = 0; i < 8; i++ )
     {
-        const unsigned a0 =  block[0+i*8] + block[4+i*8];
-        const unsigned a2 =  block[0+i*8] - block[4+i*8];
-        const unsigned a4 = (block[2+i*8]>>1) - block[6+i*8];
-        const unsigned a6 = (block[6+i*8]>>1) + block[2+i*8];
+        const unsigned a0 =  block[0+i*8] + (unsigned)block[4+i*8];
+        const unsigned a2 =  block[0+i*8] - (unsigned)block[4+i*8];
+        const unsigned a4 = (block[2+i*8]>>1) - (unsigned)block[6+i*8];
+        const unsigned a6 = (block[6+i*8]>>1) + (unsigned)block[2+i*8];
 
         const unsigned b0 = a0 + a6;
         const unsigned b2 = a2 + a4;