avcodec/vp8dsp: vp7_luma_dc_wht_c: Fix multiple runtime error: signed integer overflow: -1366381240 + -1262413604 cannot be represented in type 'int'

Fixes: 1440/clusterfuzz-testcase-minimized-5785716111966208

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

libavcodec/vp8dsp.c

@@ -53,7 +53,8 @@ static void name ## _idct_dc_add4y_c(uint8_t *dst, int16_t block[4][16],      \
 #if CONFIG_VP7_DECODER
 static void vp7_luma_dc_wht_c(int16_t block[4][4][16], int16_t dc[16])
 {
-    int i, a1, b1, c1, d1;
+    int i;
+    unsigned a1, b1, c1, d1;
     int16_t tmp[16];
 
     for (i = 0; i < 4; i++) {
@@ -61,10 +62,10 @@ static void vp7_luma_dc_wht_c(int16_t block[4][4][16], int16_t dc[16])
         b1 = (dc[i * 4 + 0] - dc[i * 4 + 2]) * 23170;
         c1 = dc[i * 4 + 1] * 12540 - dc[i * 4 + 3] * 30274;
         d1 = dc[i * 4 + 1] * 30274 + dc[i * 4 + 3] * 12540;
-        tmp[i * 4 + 0] = (a1 + d1) >> 14;
-        tmp[i * 4 + 3] = (a1 - d1) >> 14;
-        tmp[i * 4 + 1] = (b1 + c1) >> 14;
-        tmp[i * 4 + 2] = (b1 - c1) >> 14;
+        tmp[i * 4 + 0] = (int)(a1 + d1) >> 14;
+        tmp[i * 4 + 3] = (int)(a1 - d1) >> 14;
+        tmp[i * 4 + 1] = (int)(b1 + c1) >> 14;
+        tmp[i * 4 + 2] = (int)(b1 - c1) >> 14;
     }
 
     for (i = 0; i < 4; i++) {
@@ -73,10 +74,10 @@ static void vp7_luma_dc_wht_c(int16_t block[4][4][16], int16_t dc[16])
         c1 = tmp[i + 4] * 12540 - tmp[i + 12] * 30274;
         d1 = tmp[i + 4] * 30274 + tmp[i + 12] * 12540;
         AV_ZERO64(dc + i * 4);
-        block[0][i][0] = (a1 + d1 + 0x20000) >> 18;
-        block[3][i][0] = (a1 - d1 + 0x20000) >> 18;
-        block[1][i][0] = (b1 + c1 + 0x20000) >> 18;
-        block[2][i][0] = (b1 - c1 + 0x20000) >> 18;
+        block[0][i][0] = (int)(a1 + d1 + 0x20000) >> 18;
+        block[3][i][0] = (int)(a1 - d1 + 0x20000) >> 18;
+        block[1][i][0] = (int)(b1 + c1 + 0x20000) >> 18;
+        block[2][i][0] = (int)(b1 - c1 + 0x20000) >> 18;
     }
 }