Ensure padding values are positive
Ensure padding values are positive

svn path=/trunk/libcss/; revision=6233

servo/libcss,zhuyadong/libcss,servo/libcss,servo/libcss,zhuyadong/libcss,zhuyadong/libcss,prepare/netsurf_libcss,prepare/netsurf_libcss,prepare/netsurf_libcss