Fix call to ssh2_auth_pubkey() without password

Port of the previous fix for ssh2_auth_pubkey_file() to
the passwordless function introduced after that patch.

Related: https://bugs.php.net/bug.php?id=79702

Author: cweiske

ssh2.c

@@ -709,15 +709,15 @@ PHP_FUNCTION(ssh2_auth_pubkey)
 {
 	LIBSSH2_SESSION *session;
 	zval *zsession;
-	zend_string *username, *pubkey, *privkey, *passphrase;
+	zend_string *username, *pubkey, *privkey, *passphrase = NULL;
 
 	if (zend_parse_parameters(ZEND_NUM_ARGS(), "rSSS|S", &zsession,	&username, &pubkey, &privkey, &passphrase) == FAILURE) {
 		return;
 	}
 
 	SSH2_FETCH_NONAUTHENTICATED_SESSION(session, zsession);
 
-	if (libssh2_userauth_publickey_frommemory(session, ZSTR_VAL(username), ZSTR_LEN(username), ZSTR_VAL(pubkey), ZSTR_LEN(pubkey), ZSTR_VAL(privkey), ZSTR_LEN(privkey), ZSTR_VAL(passphrase))) {
+	if (libssh2_userauth_publickey_frommemory(session, ZSTR_VAL(username), ZSTR_LEN(username), ZSTR_VAL(pubkey), ZSTR_LEN(pubkey), ZSTR_VAL(privkey), ZSTR_LEN(privkey), passphrase ? ZSTR_VAL(passphrase) : NULL)) {
 		char *buf;
 		int len;
 		libssh2_session_last_error(session, &buf, &len, 0);