bug 1235965 - null check mDoc in AccessibleWrap::Shutdown() r=davidb

accessible/windows/msaa/AccessibleWrap.cpp

@@ -91,8 +91,13 @@ void
 AccessibleWrap::Shutdown()
 {
 #ifdef _WIN64
-  if (mID != kNoID)
-    static_cast<DocAccessibleWrap*>(mDoc)->RemoveID(mID);
+  if (mID != kNoID) {
+    auto doc = static_cast<DocAccessibleWrap*>(mDoc);
+    MOZ_ASSERT(doc);
+    if (doc) {
+      doc->RemoveID(mID);
+    }
+  }
 #endif
 
   Accessible::Shutdown();