Revert "libvncserver: detach client thread before ending it"

This reverts commit 8560a5a.

It caused several crashes when ending a server: client threads would
sometimes linger around until after the server was stopped and its
rfbScreen internals free()ed, with these client threads trying to access
rfbScreen and friends: 💣.

Reopens #570

Author: bk138

src/libvncserver/main.c

@@ -610,10 +610,6 @@ clientInput(void *data)
 
     rfbClientConnectionGone(cl);
 
-    /* We can't reliably join this thread, so detach it here
-       so the associated memory will be freed by the OS. */
-    THREAD_DETACH(cl->client_thread);
-
     return THREAD_ROUTINE_RETURN_VALUE;
 }
 
@@ -1208,7 +1204,10 @@ void rfbShutdownServer(rfbScreenInfoPtr screen,rfbBool disconnectClients) {
       }
 
 #ifdef LIBVNCSERVER_HAVE_LIBPTHREAD
-    if(!currentCl->screen->backgroundLoop) {
+    if(currentCl->screen->backgroundLoop) {
+      /* Wait for threads to finish. The thread has already been pipe-notified by rfbCloseClient() */
+      pthread_join(currentCl->client_thread, NULL);
+    } else {
       /*
 	In threaded mode, rfbClientConnectionGone() is called by the client-to-server thread.
 	Only need to call this here for non-threaded mode.