swscale: increase yuv2rgb table headroom

Fixes out of array access
Fixes: case2_bad_read_yuv2rgbx32.mp4
Found-by: Michal Zalewski <lcamtuf@coredump.cx>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/swscale_internal.h

@@ -39,7 +39,7 @@
 
 #define STR(s) AV_TOSTRING(s) // AV_STRINGIFY is too long
 
-#define YUVRGB_TABLE_HEADROOM 128
+#define YUVRGB_TABLE_HEADROOM 256
 
 #define MAX_FILTER_SIZE SWS_MAX_FILTER_SIZE