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/llmessage/aicurl.cpp

@@ -408,6 +408,16 @@ void initCurl(void)
   }
 }
 
+// MAIN-THREAD
+void shutdownCurl(void)
+{
+  using namespace AICurlPrivate;
+
+  DoutEntering(dc::curl, "AICurlInterface::shutdownCurl()");
+
+  BufferedCurlEasyRequest::shutdown();
+}
+
 // MAIN-THREAD
 void cleanupCurl(void)
 {
@@ -1293,6 +1303,8 @@ bool CurlEasyRequest::removeFromPerHostQueue(AICurlEasyRequest const& easy_reque
 static int const HTTP_REDIRECTS_DEFAULT = 10;
 
 LLChannelDescriptors const BufferedCurlEasyRequest::sChannels;
+LLMutex BufferedCurlEasyRequest::sResponderCallbackMutex;
+bool BufferedCurlEasyRequest::sShuttingDown = false;
 
 BufferedCurlEasyRequest::BufferedCurlEasyRequest() : mRequestTransferedBytes(0), mResponseTransferedBytes(0), mBufferEventsTarget(NULL), mStatus(HTTP_INTERNAL_ERROR_OTHER)
 {