Make mkaply's bustage fix a little safer.

content/html/document/src/nsMediaDocument.h

@@ -68,7 +68,7 @@ class nsMediaDocumentStreamListener: public nsIStreamListener
 {
 public:
   nsMediaDocumentStreamListener(nsMediaDocument *aDocument);
-  nsMediaDocumentStreamListener() {};
+  nsMediaDocumentStreamListener() { NS_NOTREACHED("should never be called"); }
   virtual ~nsMediaDocumentStreamListener();
   void SetStreamListener(nsIStreamListener *aListener);