DOMWindow::navigator should return navigator w/o frame when detached.

Source/modules/gamepad/NavigatorGamepad.cpp

Index: Source/modules/gamepad/NavigatorGamepad.cpp
diff --git a/Source/modules/gamepad/NavigatorGamepad.cpp b/Source/modules/gamepad/NavigatorGamepad.cpp
index f6673f8978ec4774810f5c0bcf4ecc3fa52876d8..00f6cb8f9f7ceb15209cc08ee86479725e53efdf 100644
--- a/Source/modules/gamepad/NavigatorGamepad.cpp
+++ b/Source/modules/gamepad/NavigatorGamepad.cpp
@@ -73,9 +73,11 @@ static void sampleGamepads(ListType* into)
 NavigatorGamepad* NavigatorGamepad::from(Document& document)
 {
     if (!document.frame() || !document.frame()->domWindow())
-        return 0;
-    Navigator& navigator = *document.frame()->domWindow()->navigator();
-    return &from(navigator);
+        return nullptr;
+    Navigator* navigator = document.frame()->domWindow()->navigator();
+    if (!navigator)
+        return nullptr;
+    return &from(*navigator);
 }
 
 NavigatorGamepad& NavigatorGamepad::from(Navigator& navigator)