Reset MBREX(search_re) in RSHUTDOWN

Reset MBREX(search_re) in RSHUTDOWN

This is going to cause a segfault if reused in the next request.
To illustrate the issue, run these two scripts in sequence with
the built-in server:

// script1.php
mb_ereg_search_init('foobar');
mb_ereg_search('foo');

// script2.php
var_dump(mb_ereg_search_init("foobar"));
var_dump(mb_ereg_search_pos());