BUG/MAJOR: cookie prefix doesn't support cookie-less servers

BUG/MAJOR: cookie prefix doesn't support cookie-less servers

Commit 827aee91 merged in 1.5-dev5 introduced a regression causing
the srv pointer to be tested twice instead of srv then srv->cookie.
The result is that if a server has no cookie in prefix mode, haproxy
will crash when trying to modify it.

Such a config is very unlikely to happen, except maybe with a backup
server, which would cause haproxy to die with the last server in the
farm.

No backport is needed, only 1.5-dev was affected.