swscale/output: Fix undefined integer overflow in yuv2rgba64_2_c_template()

Fixes: signed integer overflow: -1082982400 + -1083218484 cannot be represented in type 'int'
Fixes: 70657/clusterfuzz-testcase-minimized-ffmpeg_SWS_fuzzer-6707819712675840

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

libswscale/output.c

@@ -1150,8 +1150,8 @@ yuv2rgba64_2_c_template(SwsContext *c, const int32_t *buf[2],
     av_assert2(uvalpha <= 4096U);
 
     for (i = 0; i < ((dstW + 1) >> 1); i++) {
-        int Y1 = (buf0[i * 2]     * yalpha1  + buf1[i * 2]     * yalpha) >> 14;
-        int Y2 = (buf0[i * 2 + 1] * yalpha1  + buf1[i * 2 + 1] * yalpha) >> 14;
+        unsigned Y1 = (buf0[i * 2]     * yalpha1  + buf1[i * 2]     * yalpha) >> 14;
+        unsigned Y2 = (buf0[i * 2 + 1] * yalpha1  + buf1[i * 2 + 1] * yalpha) >> 14;
         int U  = (ubuf0[i]        * uvalpha1 + ubuf1[i]        * uvalpha - (128 << 23)) >> 14;
         int V  = (vbuf0[i]        * uvalpha1 + vbuf1[i]        * uvalpha - (128 << 23)) >> 14;
         int R, G, B;
@@ -1175,20 +1175,20 @@ yuv2rgba64_2_c_template(SwsContext *c, const int32_t *buf[2],
             A2 += 1 << 13;
         }
 
-        output_pixel(&dest[0], av_clip_uintp2(((R_B + Y1) >> 14) + (1<<15), 16));
-        output_pixel(&dest[1], av_clip_uintp2(((  G + Y1) >> 14) + (1<<15), 16));
-        output_pixel(&dest[2], av_clip_uintp2(((B_R + Y1) >> 14) + (1<<15), 16));
+        output_pixel(&dest[0], av_clip_uintp2(((int)(R_B + Y1) >> 14) + (1<<15), 16));
+        output_pixel(&dest[1], av_clip_uintp2(((int)(  G + Y1) >> 14) + (1<<15), 16));
+        output_pixel(&dest[2], av_clip_uintp2(((int)(B_R + Y1) >> 14) + (1<<15), 16));
         if (eightbytes) {
             output_pixel(&dest[3], av_clip_uintp2(A1      , 30) >> 14);
-            output_pixel(&dest[4], av_clip_uintp2(((R_B + Y2) >> 14) + (1<<15), 16));
-            output_pixel(&dest[5], av_clip_uintp2(((  G + Y2) >> 14) + (1<<15), 16));
-            output_pixel(&dest[6], av_clip_uintp2(((B_R + Y2) >> 14) + (1<<15), 16));
+            output_pixel(&dest[4], av_clip_uintp2(((int)(R_B + Y2) >> 14) + (1<<15), 16));
+            output_pixel(&dest[5], av_clip_uintp2(((int)(  G + Y2) >> 14) + (1<<15), 16));
+            output_pixel(&dest[6], av_clip_uintp2(((int)(B_R + Y2) >> 14) + (1<<15), 16));
             output_pixel(&dest[7], av_clip_uintp2(A2      , 30) >> 14);
             dest += 8;
         } else {
-            output_pixel(&dest[3], av_clip_uintp2(((R_B + Y2) >> 14) + (1<<15), 16));
-            output_pixel(&dest[4], av_clip_uintp2(((  G + Y2) >> 14) + (1<<15), 16));
-            output_pixel(&dest[5], av_clip_uintp2(((B_R + Y2) >> 14) + (1<<15), 16));
+            output_pixel(&dest[3], av_clip_uintp2(((int)(R_B + Y2) >> 14) + (1<<15), 16));
+            output_pixel(&dest[4], av_clip_uintp2(((int)(  G + Y2) >> 14) + (1<<15), 16));
+            output_pixel(&dest[5], av_clip_uintp2(((int)(B_R + Y2) >> 14) + (1<<15), 16));
             dest += 6;
         }
     }