avcodec/h264idct_template: Fix multiple runtime error: signed integer overflow

Fixes: 677/clusterfuzz-testcase-6635120628858880

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Reviewed-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

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 int z0=  block[i + 4*0]     +  block[i + 4*2];
-        const int z1=  block[i + 4*0]     -  block[i + 4*2];
-        const int z2= (block[i + 4*1]>>1) -  block[i + 4*3];
-        const int z3=  block[i + 4*1]     + (block[i + 4*3]>>1);
+        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);
 
         block[i + 4*0]= z0 + z3;
         block[i + 4*1]= z1 + z2;
@@ -52,15 +52,15 @@ void FUNCC(ff_h264_idct_add)(uint8_t *_dst, int16_t *_block, int stride)
     }
 
     for(i=0; i<4; i++){
-        const int z0=  block[0 + 4*i]     +  block[2 + 4*i];
-        const int z1=  block[0 + 4*i]     -  block[2 + 4*i];
-        const int z2= (block[1 + 4*i]>>1) -  block[3 + 4*i];
-        const int z3=  block[1 + 4*i]     + (block[3 + 4*i]>>1);
-
-        dst[i + 0*stride]= av_clip_pixel(dst[i + 0*stride] + ((z0 + z3) >> 6));
-        dst[i + 1*stride]= av_clip_pixel(dst[i + 1*stride] + ((z1 + z2) >> 6));
-        dst[i + 2*stride]= av_clip_pixel(dst[i + 2*stride] + ((z1 - z2) >> 6));
-        dst[i + 3*stride]= av_clip_pixel(dst[i + 3*stride] + ((z0 - z3) >> 6));
+        const SUINT z0=  block[0 + 4*i]     +  (SUINT)block[2 + 4*i];
+        const SUINT z1=  block[0 + 4*i]     -  (SUINT)block[2 + 4*i];
+        const SUINT z2= (block[1 + 4*i]>>1) -  (SUINT)block[3 + 4*i];
+        const SUINT z3=  block[1 + 4*i]     + (SUINT)(block[3 + 4*i]>>1);
+
+        dst[i + 0*stride]= av_clip_pixel(dst[i + 0*stride] + ((int)(z0 + z3) >> 6));
+        dst[i + 1*stride]= av_clip_pixel(dst[i + 1*stride] + ((int)(z1 + z2) >> 6));
+        dst[i + 2*stride]= av_clip_pixel(dst[i + 2*stride] + ((int)(z1 - z2) >> 6));
+        dst[i + 3*stride]= av_clip_pixel(dst[i + 3*stride] + ((int)(z0 - z3) >> 6));
     }
 
     memset(block, 0, 16 * sizeof(dctcoef));