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());

ext/mbstring/php_mbregex.c

@@ -149,6 +149,7 @@ PHP_RSHUTDOWN_FUNCTION(mb_regex)
 		ZVAL_UNDEF(&MBREX(search_str));
 	}
 	MBREX(search_pos) = 0;
+	MBREX(search_re) = NULL;
 
 	if (MBREX(search_regs) != NULL) {
 		onig_region_free(MBREX(search_regs), 1);