fix a segfault if flushing cookies with no CookieInfo set

[m6w6]

a52e46f removed a NULL check, reintroduced here.

[bagder]

I've struggled to understand how we end up with data->cookies being NULL while data->set.str[STRING_COOKIEJAR] is set. It seeems to indicate a problem somewhere and this just paints over that symptom.

I think the explanation is related to cookie sharing and changing that share in a specific handle. Like here:

curl/lib/setopt.c

Line 1995 in 90236ed

data->cookies = NULL;

[bagder]

Thanks!

[m6w6]

I'm sorry, that I cannot give more insight. What I can say, though, is that I always use multi and I always use share and I always enable cookie sharing and I always flush cookies, but I do not always use a cookie file/jar.