Cookie Replacement Use-After-Free Vulnerability

Summary: The cookie replacement logic in `lib/cookie.c` contains a use-after-free vulnerability in the `replace_existing()` function. The function modifies a linked list while iterating over it, creating potential for memory corruption in concurrent or complex cookie operations.

static bool replace_existing(struct Curl_easy *data,
                             struct Cookie *co,
                             struct CookieInfo *ci,
                             bool secure,
                             bool *replacep)
{
    bool replace_old = FALSE;
    struct Curl_llist_node …