LibWeb: Fail to parse cookie date when date does not exist

Previously, the cookie date validation did not validate days in the
context of the month and year, resulting in dates that do not exist to
be successfully parsed (e.g. February 31st). We now validate that the
day does not exceed the number of days for the given month and year,
taking leap years into account.

Userland/Libraries/LibWeb/Cookie/ParsedCookie.cpp

@@ -345,7 +345,9 @@ Optional<UnixDateTime> parse_date_time(StringView date_string)
     // 6. Let the parsed-cookie-date be the date whose day-of-month, month, year, hour, minute, and second (in UTC) are the
     //    day-of-month-value, the month-value, the year-value, the hour-value, the minute-value, and the second-value, respectively.
     //    If no such date exists, abort these steps and fail to parse the cookie-date.
-    // FIXME: Fail on dates that do not exist.
+    if (day_of_month > static_cast<unsigned int>(days_in_month(year, month)))
+        return {};
+
     // FIXME: This currently uses UNIX time, which is not equivalent to UTC due to leap seconds.
     auto parsed_cookie_date = UnixDateTime::from_unix_time_parts(year, month, day_of_month, hour, minute, second, 0);