Large cookies cause buffer overflow

[aph3rson]

When using -j, the cookie header can be populated by slowhttptest when sending the request.

However, there's an undocumented 1024-byte limit on the size of this header: https://github.com/shekyan/slowhttptest/blob/6e316be98f562dd129a76cb228faae83217030a8/src/slowhttptestmain.cc#L155

For applications that use large authorization cookies (e.g. JWTs), the cookie may exceed 1024 bytes. Using these cookies on the command line will trigger buffer overflow detection - while you're not overflowing the buffer, you're also not writing a null byte.

Improvements here may include:

• documenting a max length of 1024 for the Cookie (and Accept) headers.
• fail if the -j option is longer than this max length
• improving the cookie variable to allocate a variable-length buffer of just-enough space for the provided cookies