cookie: cookie parser out of boundary memory access

The internal libcurl function called sanitize_cookie_path() that cleans
up the path element as given to it from a remote site or when read from
a file, did not properly validate the input. If given a path that
consisted of a single double-quote, libcurl would index a newly
allocated memory area with index -1 and assign a zero to it, thus
destroying heap memory it wasn't supposed to.

CVE-2015-3145

Bug: http://curl.haxx.se/docs/adv_20150422C.html
Reported-by: Hanno Böck