avcodec/h264idct_template: Fix integer overflow in ff_h264_idct_add()

Fixes: runtime error: signed integer overflow: 26215360 + 2121330944 cannot be represented in type 'int'
Fixes: 2809/clusterfuzz-testcase-minimized-4785181833560064

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg

libavcodec/h264idct_template.c

@@ -40,10 +40,10 @@ void FUNCC(ff_h264_idct_add)(uint8_t *_dst, int16_t *_block, int stride)
     block[0] += 1 << 5;
 
     for(i=0; i<4; i++){
-        const SUINT z0=  block[i + 4*0]     +  block[i + 4*2];
-        const SUINT z1=  block[i + 4*0]     -  block[i + 4*2];
-        const SUINT z2= (block[i + 4*1]>>1) -  block[i + 4*3];
-        const SUINT z3=  block[i + 4*1]     + (block[i + 4*3]>>1);
+        const SUINT z0=  block[i + 4*0]     +  (unsigned)block[i + 4*2];
+        const SUINT z1=  block[i + 4*0]     -  (unsigned)block[i + 4*2];
+        const SUINT z2= (block[i + 4*1]>>1) -  (unsigned)block[i + 4*3];
+        const SUINT z3=  block[i + 4*1]     + (unsigned)(block[i + 4*3]>>1);
 
         block[i + 4*0]= z0 + z3;
         block[i + 4*1]= z1 + z2;