Add null checks in navigator.serviceWorker access to fix possible crash

Source/modules/serviceworkers/ServiceWorkerContainerClient.cpp

Index: Source/modules/serviceworkers/ServiceWorkerContainerClient.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorkerContainerClient.cpp b/Source/modules/serviceworkers/ServiceWorkerContainerClient.cpp
index fca53dfd79bb8bcd49e3c88883c6e0786021bc9f..3d546eb37d002b50cbac943f6c477c181c806779 100644
--- a/Source/modules/serviceworkers/ServiceWorkerContainerClient.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerContainerClient.cpp
@@ -36,8 +36,10 @@ ServiceWorkerContainerClient* ServiceWorkerContainerClient::from(ExecutionContex
         if (client)
             return client;
 
-        // If it's not provided yet create it lazily.
-        ASSERT(document->frame());
+        if (!document->frame())
+            return 0;
+
+        // If it's not provided yet, create it lazily.
         document->page()->provideSupplement(ServiceWorkerContainerClient::supplementName(), ServiceWorkerContainerClient::create(document->frame()->loader().client()->createServiceWorkerProvider()));
         return static_cast<ServiceWorkerContainerClient*>(Supplement<Page>::from(document->page(), supplementName()));
     }