Add nil check for FrontendConfig in checkProxy()

If you delete the FrontendConfig before the redirectUm is cleaned up,
the FrontendConfig will be nil and l7.rediretUm will not be, leading to an "unrecoverable" crash loop.
The only fix is to manually update the target proxy's link to the original urlmap.

pkg/loadbalancers/target_proxies.go

@@ -38,7 +38,10 @@ func (l7 *L7) checkProxy() (err error) {
 	// TODO(shance): move to translator
 	var umName string
 	if flags.F.EnableFrontendConfig {
-		if l7.redirectUm != nil && l7.runtimeInfo.FrontendConfig.Spec.RedirectToHttps != nil && l7.runtimeInfo.FrontendConfig.Spec.RedirectToHttps.Enabled {
+		if l7.redirectUm != nil &&
+			l7.runtimeInfo.FrontendConfig != nil &&
+			l7.runtimeInfo.FrontendConfig.Spec.RedirectToHttps != nil &&
+			l7.runtimeInfo.FrontendConfig.Spec.RedirectToHttps.Enabled {
 			umName = l7.redirectUm.Name
 		} else {
 			umName = l7.um.Name