http: never send 'Cookie: (null)' to the server

If a domain has some cookies set, but matching the cookie fails due to
the port being different, get_cookies() succeeds, but sets cookies to
NULL. The caller of get_cookies() didn't check for the NULL value.

This also avoids passing NULL to libc string functions, which is
undefined behavior

Fixes Ticket2180

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>