BUILD: backend: fix 3 build warnings related to null-deref at -Wextra

These ones are not valid either since the checks are performed a few
lines above the call. Let's switch to __objt_server() instead.
diff --git a/src/backend.c b/src/backend.c
index 3445a9d..e66ee4a 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -795,8 +795,8 @@
 		if (!(s->flags & SF_ASSIGNED))
 			return SRV_STATUS_INTERNAL;
 
-		srv_conn->addr.to = objt_server(s->target)->addr;
-		set_host_port(&srv_conn->addr.to, objt_server(s->target)->svc_port);
+		srv_conn->addr.to = __objt_server(s->target)->addr;
+		set_host_port(&srv_conn->addr.to, __objt_server(s->target)->svc_port);
 
 		if (!is_addr(&srv_conn->addr.to) && cli_conn) {
 			/* if the server has no address, we use the same address
@@ -815,7 +815,7 @@
 
 		/* if this server remaps proxied ports, we'll use
 		 * the port the client connected to with an offset. */
-		if ((objt_server(s->target)->flags & SRV_F_MAPPORTS) && cli_conn) {
+		if ((__objt_server(s->target)->flags & SRV_F_MAPPORTS) && cli_conn) {
 			int base_port;
 
 			conn_get_to_addr(cli_conn);
@@ -1761,7 +1761,7 @@
 		return 0;
 
 	smp->data.type = SMP_T_SINT;
-	smp->data.u.sint = objt_server(smp->strm->target)->puid;
+	smp->data.u.sint = __objt_server(smp->strm->target)->puid;
 
 	return 1;
 }