PR #1723 - fix for crash in mysqli_connect

Hi all, Can someone take a look at this PR to fix a crash with mysqli_connect arising with a mysqlnd refactor in e81ecc80c in master from November: https://github.com/php/php-src/pull/1723 When attempting to mysqlI_connect(‘localhost’, ‘user’, ‘pass), the mysqlnd_conn_data::get_scheme introduced in e81ecc80c was no longer able to update the socket_or_pipe in mysqlnd_conn_data::connect with the socket filename in the event hostname == localhost. This causes a crash in mysqlnd_conn_data::connect when an attempt is made to duplicate the string to store it in conn->unix_socket.s. Thanks! -John