avcodec/svq3: Fix multiple runtime error: signed integer overflow: -237341 * 24552 cannot be represented in type 'int'

Fixes: 1429/clusterfuzz-testcase-minimized-5959951610544128

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

libavcodec/svq3.c

@@ -281,16 +281,16 @@ static void svq3_add_idct_c(uint8_t *dst, int16_t *block,
     }
 
     for (i = 0; i < 4; i++) {
-        const int z0 = 13 * (block[i + 4 * 0] +      block[i + 4 * 2]);
-        const int z1 = 13 * (block[i + 4 * 0] -      block[i + 4 * 2]);
-        const int z2 =  7 *  block[i + 4 * 1] - 17 * block[i + 4 * 3];
-        const int z3 = 17 *  block[i + 4 * 1] +  7 * block[i + 4 * 3];
+        const unsigned z0 = 13 * (block[i + 4 * 0] +      block[i + 4 * 2]);
+        const unsigned z1 = 13 * (block[i + 4 * 0] -      block[i + 4 * 2]);
+        const unsigned z2 =  7 *  block[i + 4 * 1] - 17 * block[i + 4 * 3];
+        const unsigned z3 = 17 *  block[i + 4 * 1] +  7 * block[i + 4 * 3];
         const int rr = (dc + 0x80000);
 
-        dst[i + stride * 0] = av_clip_uint8(dst[i + stride * 0] + ((z0 + z3) * qmul + rr >> 20));
-        dst[i + stride * 1] = av_clip_uint8(dst[i + stride * 1] + ((z1 + z2) * qmul + rr >> 20));
-        dst[i + stride * 2] = av_clip_uint8(dst[i + stride * 2] + ((z1 - z2) * qmul + rr >> 20));
-        dst[i + stride * 3] = av_clip_uint8(dst[i + stride * 3] + ((z0 - z3) * qmul + rr >> 20));
+        dst[i + stride * 0] = av_clip_uint8(dst[i + stride * 0] + ((int)((z0 + z3) * qmul + rr) >> 20));
+        dst[i + stride * 1] = av_clip_uint8(dst[i + stride * 1] + ((int)((z1 + z2) * qmul + rr) >> 20));
+        dst[i + stride * 2] = av_clip_uint8(dst[i + stride * 2] + ((int)((z1 - z2) * qmul + rr) >> 20));
+        dst[i + stride * 3] = av_clip_uint8(dst[i + stride * 3] + ((int)((z0 - z3) * qmul + rr) >> 20));
     }
 
     memset(block, 0, 16 * sizeof(int16_t));