Fix crash in LLTextureFetch::getWorker upon viewer exit.

This is now necessary since the curl thread no longer syncs with the
main thread: it is possible that a request finishes after a texture
fetch thread was shot down but before curl was stopped, and curl
calling BufferedCurlEasyRequest::processOutput while objects that the
responder uses were already destructed (most notably
LLTextureFetch itself).

indra/newview/llappviewer.cpp

@@ -1758,6 +1758,8 @@ bool LLAppViewer::cleanup()
 			LLViewerMedia::saveCookieFile();
 	// Stop the plugin read thread if it's running.
 	LLPluginProcessParent::setUseReadThread(false);
+	// Stop curl responder call backs.
+	AICurlInterface::shutdownCurl();
 
 	llinfos << "Shutting down Threads" << llendflush;