crypto: fix X509 cert memory leak in GetPeerCert

`SSL_get_peer_certificate` returns referenced X509 object, we should
decrement the reference count once it is not needed.

Fix joyent/node#8674
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/node-forward/node/pull/56

src/node_crypto.cc

@@ -1403,9 +1403,10 @@ void SSLWrap<Base>::GetPeerCertificate(
     info->Set(env->issuercert_string(), info);
 
   CHECK_NE(cert, nullptr);
-  X509_free(cert);
 
  done:
+  if (cert != nullptr)
+    X509_free(cert);
   if (peer_certs != nullptr)
     sk_X509_pop_free(peer_certs, X509_free);
   if (result.IsEmpty())