swscale/output: Fix undefined overflow in yuv2rgba64_full_X_c_template()

Fixes: signed integer overflow: -1082982400 + -1195645138 cannot be represented in type 'int'
Fixes: 376136843/clusterfuzz-testcase-minimized-ffmpeg_SWS_fuzzer-4791844321427456

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

@@ -1355,9 +1355,9 @@ yuv2rgba64_full_X_c_template(SwsInternal *c, const int16_t *lumFilter,
         B =                            U * c->yuv2rgb_u2b_coeff;
 
         // 8bit: 30 - 22 = 8bit, 16bit: 30bit - 14 = 16bit
-        output_pixel(&dest[0], av_clip_uintp2(((R_B + Y)>>14) + (1<<15), 16));
-        output_pixel(&dest[1], av_clip_uintp2(((  G + Y)>>14) + (1<<15), 16));
-        output_pixel(&dest[2], av_clip_uintp2(((B_R + Y)>>14) + (1<<15), 16));
+        output_pixel(&dest[0], av_clip_uintp2(((int)(R_B + (unsigned)Y)>>14) + (1<<15), 16));
+        output_pixel(&dest[1], av_clip_uintp2(((int)(  G + (unsigned)Y)>>14) + (1<<15), 16));
+        output_pixel(&dest[2], av_clip_uintp2(((int)(B_R + (unsigned)Y)>>14) + (1<<15), 16));
         if (eightbytes) {
             output_pixel(&dest[3], av_clip_uintp2(A, 30) >> 14);
             dest += 4;