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.

src/server.c

@@ -5299,6 +5299,10 @@ static void srv_cleanup_connections(struct server *srv)
 	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 */
 	for (i = tid;;) {
 		did_remove = 0;