src: fix SetClientCertEngine() nullptr dereference

Introduced in commit 6ee985f ("tls: implement clientCertEngine
option") which was merged November 11.

PR-URL: #16965
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: bnoordhuis

src/node_crypto.cc

@@ -1348,7 +1348,8 @@ void SecureContext::SetClientCertEngine(
   CHECK_EQ(args.Length(), 1);
   CHECK(args[0]->IsString());
 
-  SecureContext* sc = Unwrap<SecureContext>(args.This());
+  SecureContext* sc;
+  ASSIGN_OR_RETURN_UNWRAP(&sc, args.Holder());
 
   MarkPopErrorOnReturn mark_pop_error_on_return;