BUG/MINOR: srv: do not init address if backend is disabled

BUG/MINOR: srv: do not init address if backend is disabled

Do not proceed on init_addr if the backend of the server is marked as
disabled. When marked as disabled, the server is not fully initialized
and some operation must be avoided to prevent segfault. It is correct
because there is no way to activate a disabled backend.

This fixes the github issue #1031.
This should be backported to 2.2.

(cherry picked from commit e3c419296211f08209300b9a06dc17467a6cb315)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>