BUG/MINOR: srv: do not cleanup idle conns if pool max is null

If a server is configured to not have any idle conns, returns immediatly
from srv_cleanup_connections. This avoids a segfault when a server is
configured with pool-max-conn to 0.

This should be backported up to 2.2.

(cherry picked from commit 10d5c3172b366e48b5851a20de9b68f0d5495b92)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/server.c b/src/server.c
index da1059c..79de104 100644
--- a/src/server.c
+++ b/src/server.c
@@ -5221,6 +5221,10 @@
 	int did_remove;
 	int i;
 
+	/* nothing to do if pool-max-conn is null */
+	if (!srv->max_idle_conns)
+		return;
+
 	/* check all threads starting with ours */
 	HA_SPIN_LOCK(OTHER_LOCK, &idle_conn_srv_lock);
 	for (i = tid;;) {