swscale/yuv2rgb: clip cy, avoid division by 0 with 0 contrast

Found-by: durandal_1707
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/yuv2rgb.c

@@ -768,10 +768,10 @@ av_cold int ff_yuv2rgb_c_init_tables(SwsContext *c, const int inv_table[4],
     c->yuv2rgb_u2b_coeff = (int16_t)roundToInt16(cbu << 13);
 
     //scale coefficients by cy
-    crv = ((crv << 16) + 0x8000) / cy;
-    cbu = ((cbu << 16) + 0x8000) / cy;
-    cgu = ((cgu << 16) + 0x8000) / cy;
-    cgv = ((cgv << 16) + 0x8000) / cy;
+    crv = ((crv << 16) + 0x8000) / FFMAX(cy, 1);
+    cbu = ((cbu << 16) + 0x8000) / FFMAX(cy, 1);
+    cgu = ((cgu << 16) + 0x8000) / FFMAX(cy, 1);
+    cgv = ((cgv << 16) + 0x8000) / FFMAX(cy, 1);
 
     av_freep(&c->yuvTable);