[MINOR] allow a proxy instance to run without any server (useful for stats)

ROADMAP

@@ -18,6 +18,8 @@
         stats auth user3:pass3
         stats scope <px_id> | '.'
 
+ + allow server-less proxies (for stats)
+
  - separate timeout controls
 
  - option 'abortonclose' : if the session is queued or being connecting
@@ -33,8 +35,6 @@
      srv->effective_maxconn =
           max(srv->maxconn * px->nbsess / px->maxconn, srv->minconn)
 
- - allow server-less proxies (for stats)
-
 1.3 :
  - handle half-closed connections better (cli/srv would not distinguish
    DATA/SHUTR/SHUTW, it would be a session flag which would tell shutr/shutw).

haproxy.c

@@ -9233,11 +9233,13 @@ int readcfgfile(char *file) {
 		      file, curproxy->id);
 		cfgerr++;
 	    }
+#ifdef WE_DONT_SUPPORT_SERVERLESS_LISTENERS
 	    else if (curproxy->srv == NULL) {
 		Alert("parsing %s : listener %s needs at least 1 server in balance mode.\n",
 		      file, curproxy->id);
 		cfgerr++;
 	    }
+#endif
 	    else if (*(int *)&curproxy->dispatch_addr.sin_addr != 0) {
 		Warning("parsing %s : dispatch address of listener %s will be ignored in balance mode.\n",
 			file, curproxy->id);