mdns_init: avoid null reference access

- when avahi client creation fails with mdns_client=null, use avahi_strerror(error) instead of MDNSERR
  to avoid access to mdns_client->error.

src/mdns_avahi.c

@@ -1044,7 +1044,7 @@ mdns_init(void)
 				 client_callback, NULL, &error);
   if (!mdns_client)
     {
-      DPRINTF(E_WARN, L_MDNS, "mdns_init: Could not create Avahi client: %s\n", MDNSERR);
+      DPRINTF(E_WARN, L_MDNS, "mdns_init: Could not create Avahi client: %s\n", avahi_strerror(error));
 
       return -1;
     }