ssl: avoid directly storing String object in NPN callback

On the server side, the serialized list of protocols is stored in
SSL_CTX as a String object reference. We utilize a hidden instance
variable to prevent it from being GC'ed, but this is not enough because
it can also be relocated by GC.compact.

ext/openssl/ossl_ssl.c

@@ -698,7 +698,7 @@ static int
 ssl_npn_advertise_cb(SSL *ssl, const unsigned char **out, unsigned int *outlen,
 		     void *arg)
 {
-    VALUE protocols = (VALUE)arg;
+    VALUE protocols = rb_attr_get((VALUE)arg, id_npn_protocols_encoded);
 
     *out = (const unsigned char *) RSTRING_PTR(protocols);
     *outlen = RSTRING_LENINT(protocols);
@@ -916,7 +916,7 @@ ossl_sslctx_setup(VALUE self)
     if (!NIL_P(val)) {
 	VALUE encoded = ssl_encode_npn_protocols(val);
 	rb_ivar_set(self, id_npn_protocols_encoded, encoded);
-	SSL_CTX_set_next_protos_advertised_cb(ctx, ssl_npn_advertise_cb, (void *)encoded);
+	SSL_CTX_set_next_protos_advertised_cb(ctx, ssl_npn_advertise_cb, (void *)self);
 	OSSL_Debug("SSL NPN advertise callback added");
     }
     if (RTEST(rb_attr_get(self, id_i_npn_select_cb))) {