Avoid ubsan warning due to memcpy null

This showed up in ext/mysqli/tests/mysqli_change_user.phpt on azure
today. Not seeing it locally though, and also not sure why it decided
to show up now...

Author: nikic

ext/mysqlnd/mysqlnd_wireprotocol.c

@@ -2144,7 +2144,9 @@ size_t php_mysqlnd_cached_sha2_result_write(MYSQLND_CONN_DATA * conn, void * _pa
 		int1store(buffer + MYSQLND_HEADER_SIZE, '\2');
 		sent = pfc->data->m.send(pfc, vio, buffer, 1, stats, error_info);
 	} else {
-		memcpy(buffer + MYSQLND_HEADER_SIZE, packet->password, packet->password_len);
+		if (packet->password_len != 0) {
+			memcpy(buffer + MYSQLND_HEADER_SIZE, packet->password, packet->password_len);
+		}
 		sent = pfc->data->m.send(pfc, vio, buffer, packet->password_len, stats, error_info);
 	}