BUG/MEDIUM: server: segv when adding server with hostname from CLI

When calling 'add server' with a hostname from the cli (runtime),
str2sa_range() does not resolve hostname because it is purposely
called without PA_O_RESOLVE flag.

This leads to 'srv->addr_node.key' being NULL. According to Willy it
is fine behavior, as long as we handle it properly, and is already
handled like this in srv_set_addr_desc().

This patch fixes GH #1865 by adding an extra check before inserting
'srv->addr_node' into 'be->used_server_addr'. Insertion and removal
will be skipped if 'addr_node.key' is NULL.

It must be backported to 2.6 and 2.5 only.
diff --git a/src/server.c b/src/server.c
index debbd71..f35190d 100644
--- a/src/server.c
+++ b/src/server.c
@@ -4839,7 +4839,9 @@
 	/* insert the server in the backend trees */
 	eb32_insert(&be->conf.used_server_id, &srv->conf.id);
 	ebis_insert(&be->conf.used_server_name, &srv->conf.name);
-	ebis_insert(&be->used_server_addr, &srv->addr_node);
+	/* addr_node.key could be NULL if FQDN resolution is postponed (ie: add server from cli) */
+	if (srv->addr_node.key)
+		ebis_insert(&be->used_server_addr, &srv->addr_node);
 
 	thread_release();
 
@@ -5003,7 +5005,8 @@
 	/* remove srv from addr_node tree */
 	eb32_delete(&srv->conf.id);
 	ebpt_delete(&srv->conf.name);
-	ebpt_delete(&srv->addr_node);
+	if (srv->addr_node.key)
+		ebpt_delete(&srv->addr_node);
 
 	/* remove srv from idle_node tree for idle conn cleanup */
 	eb32_delete(&srv->idle_node);