cli: avoid attempting to setup timeout on NULL proxy

mm_manager_get_proxy() may actually return NULL if creating the
internal GDBusProxy for the Manager interface fails.

cli/mmcli.c

@@ -188,7 +188,8 @@ mmcli_force_sync_operation (void)
 void
 mmcli_force_operation_timeout (GDBusProxy *proxy)
 {
-    g_dbus_proxy_set_default_timeout (proxy, timeout * 1000);
+    if (proxy)
+        g_dbus_proxy_set_default_timeout (proxy, timeout * 1000);
 }
 
 gint