BUG/MINOR: server: fix dynamic server leak with check on failed init

BUG/MINOR: server: fix dynamic server leak with check on failed init

If a dynamic server is added with check or agent-check, its refcount is
incremented after server keyword parsing. However, if add server fails
at a later stage, refcount is only decremented once, which prevented the
server to be fully released.

This causes a leak with a server which is detached from most of the
lists but still exits in the system.

This bug is considered minor as only a few conditions may cause a
failure in add server after check/agent-check initialization. This is
the case if there is a naming collision or the dynamic ID cannot be
generated.

To fix this, simply decrement server refcount on add server error path
if either check and/or agent-check are flagged as activated.

This bug is related to github issue #2733. Thanks to Chris Staite who
first found the leak.

This must be backported up to 2.6.

(cherry picked from commit 116178563c2fb57e28a76838cf85c4858b185b76)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>