Thread: Avoid a NULL dereference after failed initialisation.

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10493)

crypto/initthread.c

@@ -380,6 +380,8 @@ static int init_thread_deregister(void *index, int all)
     int i;
 
     gtr = get_global_tevent_register();
+    if (gtr == NULL)
+        return 0;
     if (!all)
         CRYPTO_THREAD_write_lock(gtr->lock);
     for (i = 0; i < sk_THREAD_EVENT_HANDLER_PTR_num(gtr->skhands); i++) {