Cookie Max-Age Integer Overflow Vulnerability

Summary: The cookie parsing code in `lib/cookie.c` contains an integer overflow vulnerability when processing the `Max-Age` attribute of HTTP cookies. The vulnerable code attempts to add the max-age value to the current timestamp without adequate overflow protection

else if(CURL_OFF_T_MAX - now < co->expires)
    /* would overflow */
    co->expires = CURL_OFF_T_MAX;
else
    co->expires += now;