[mod_fastcgi] fix segfault if all backends down (fixes #2768)

(thx HenrikHolst)

x-ref:
  "mod_fastcgi segfault in lighttpd-1.4.43"
  https://redmine.lighttpd.net/issues/2768

src/mod_fastcgi.c

@@ -1684,6 +1684,8 @@ static fcgi_extension_host * fcgi_extension_host_get(server *srv, connection *co
 					"on", extension->key,
 					"are down.");
 		}
+
+		return NULL;
 	}
 
 	/* found a server */