MEDIUM: mworker: does not create the CLI proxy when no listener

Does not create the CLI proxy if no -S argument was specified. It
prevents a warning that says that the MASTER proxy does not have any
bind option.
diff --git a/src/cli.c b/src/cli.c
index 32eaf46..e20e0c5 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -2105,7 +2105,8 @@
  */
 void mworker_cli_proxy_stop()
 {
-	stop_proxy(mworker_proxy);
+	if (mworker_proxy)
+		stop_proxy(mworker_proxy);
 }
 
 /*