r=bienvenu. Don't fail if there's no unread thread.

mailnews/base/src/nsMsgViewNavigationService.cpp

@@ -1132,6 +1132,8 @@ nsresult nsMsgViewNavigationService::GetNextMessageByThread(nsIDOMXULElement *st
 	if(NS_FAILED(rv))
 		return rv;
 
+	if(nextTopMessage)
+	{
 		nsCOMPtr<nsIDOMXULElement> nextMessageElement;
 
 		rv = FindNextInThread(info->type, info->tree, nextTopMessage, info->rdfService,
@@ -1144,11 +1146,10 @@ nsresult nsMsgViewNavigationService::GetNextMessageByThread(nsIDOMXULElement *st
 			rv = nextMessageElement->QueryInterface(NS_GET_IID(nsIDOMNode), (void**)nextMessage);
 			return rv;
 		}
-	else
-	{
-		return NS_OK;
 	}
 
+	return NS_OK;
+
 }
 
 //Given a message, searches from siblings onward to find the next message. This is for the threaded view.