asyn-thread: fix curl_global_cleanup crash in Windows

- Make sure that asynchronous resolves handled by Winsock are stopped
  before WSACleanup is called.

This is implemented by ensuring that when Curl_resolver_kill is called
(eg via multi_done) it will cancel the Winsock asynchronous resolve and
wait for the cancellation to complete. Winsock runs the asynchronous
completion routine immediately when a resolve is canceled.

Prior to this change it was possible that during curl_global_cleanup
"a DNS resolver thread created by GetAddrInfoExW did not terminate yet,
however curl is already shutting down, deinitializing Winsock with
WSACleanup() leading to an access violation."

Background:

If libcurl is built with the asynchronous threaded resolver option for
Windows then it resolves in one of two ways. For Windows 8.1 and later,
libcurl resolves by using the Winsock asynchronous resolver which does
its own thread management. For older versions of Windows, libcurl
resolves by creating a separate thread that calls getaddrinfo. This
change only affects the former and it's already handled for the latter.

Reported-by: Ch40zz@users.noreply.github.com

Fixes #13509
Closes #13518

lib/asyn-thread.c

@@ -554,18 +554,23 @@ static void destroy_async_data(struct Curl_async *async)
 
     if(!done) {
 #ifdef _WIN32
-      if(td->complete_ev)
+      if(td->complete_ev) {
         CloseHandle(td->complete_ev);
-      else
+        td->complete_ev = NULL;
+      }
 #endif
-      Curl_thread_destroy(td->thread_hnd);
+      if(td->thread_hnd != curl_thread_t_null) {
+        Curl_thread_destroy(td->thread_hnd);
+        td->thread_hnd = curl_thread_t_null;
+      }
     }
     else {
 #ifdef _WIN32
       if(td->complete_ev) {
         Curl_GetAddrInfoExCancel(&td->tsd.w8.cancel_ev);
         WaitForSingleObject(td->complete_ev, INFINITE);
         CloseHandle(td->complete_ev);
+        td->complete_ev = NULL;
       }
 #endif
       if(td->thread_hnd != curl_thread_t_null)
@@ -713,6 +718,7 @@ static CURLcode thread_wait_resolv(struct Curl_easy *data,
   if(td->complete_ev) {
     WaitForSingleObject(td->complete_ev, INFINITE);
     CloseHandle(td->complete_ev);
+    td->complete_ev = NULL;
     if(entry)
       result = getaddrinfo_complete(data);
   }
@@ -754,6 +760,13 @@ void Curl_resolver_kill(struct Curl_easy *data)
   /* If we're still resolving, we must wait for the threads to fully clean up,
      unfortunately.  Otherwise, we can simply cancel to clean up any resolver
      data. */
+#ifdef _WIN32
+  if(td && td->complete_ev) {
+    Curl_GetAddrInfoExCancel(&td->tsd.w8.cancel_ev);
+    (void)thread_wait_resolv(data, NULL, FALSE);
+  }
+  else
+#endif
   if(td && td->thread_hnd != curl_thread_t_null
      && (data->set.quick_exit != 1L))
     (void)thread_wait_resolv(data, NULL, FALSE);

lib/curl_threads.c

@@ -131,7 +131,8 @@ curl_thread_t Curl_thread_create(unsigned int (CURL_STDCALL *func) (void *),
 
 void Curl_thread_destroy(curl_thread_t hnd)
 {
-  CloseHandle(hnd);
+  if(hnd != curl_thread_t_null)
+    CloseHandle(hnd);
 }
 
 int Curl_thread_join(curl_thread_t *hnd)