MINOR: server-state: Don't load server-state file for serverless proxies

Just a minor improvement. Proxies with no server are now ignored early. It
may happens for listeners for instance.
diff --git a/src/server_state.c b/src/server_state.c
index e3d2854..302ffb3 100644
--- a/src/server_state.c
+++ b/src/server_state.c
@@ -816,7 +816,7 @@
 		struct eb_root local_state_tree = EB_ROOT_UNIQUE;
 
 		/* servers are only in backends */
-		if (!(curproxy->cap & PR_CAP_BE))
+		if (!(curproxy->cap & PR_CAP_BE) || !curproxy->srv)
 			continue; /* next proxy */
 
 		/* No server-state file for this proxy */