http: do the cookie list access under lock

http: do the cookie list access under lock

A previous refactor of cookie logic changed Curl_cookie_getlist to no
longer return a list of copied cookies, but instead return a linked list
pointing to existing cookies. The returned linked list is accessed
outside of the scope of the cookie share lock in http_cookies, which
leads to issues if the shared cookie list is modified at the same time.
This is the relevant commit: be39ed1

Closes #18457