Fix leaks with multiple calls to DatePeriod iterator current()

Destroy the old value first. We can't skip recreating the value because
the object may have been changed in between calls.

Author: nielsdos

ext/date/php_date.c

@@ -1606,7 +1606,8 @@ static zval *date_period_it_current_data(zend_object_iterator *iter)
 	timelib_time   *it_time = object->current;
 	php_date_obj   *newdateobj;
 
-	/* Create new object */
+	/* Create new object. */
+	zval_ptr_dtor(&iterator->current);
 	php_date_instantiate(get_base_date_class(object->start_ce), &iterator->current);
 	newdateobj = Z_PHPDATE_P(&iterator->current);
 	newdateobj->time = timelib_time_ctor();

ext/date/tests/multiple_calls_date_period_iterator_current.phpt

@@ -0,0 +1,42 @@
+--TEST--
+Multiple calls to DatePeriod iterator current() leak objects
+--FILE--
+<?php
+$start = new DateTime('2018-12-31 00:00:00');
+$end   = new DateTime('2019-12-31 00:00:00');
+
+$interval = new DateInterval('P1M');
+$period = new DatePeriod($start, $interval, 1);
+
+$iter = $period->getIterator();
+var_dump($iter->current());
+var_dump($iter->current());
+$iter->current()->setTimestamp(0);
+var_dump($iter->current());
+
+?>
+--EXPECT--
+object(DateTime)#9 (3) {
+  ["date"]=>
+  string(26) "2018-12-31 00:00:00.000000"
+  ["timezone_type"]=>
+  int(3)
+  ["timezone"]=>
+  string(3) "UTC"
+}
+object(DateTime)#9 (3) {
+  ["date"]=>
+  string(26) "2018-12-31 00:00:00.000000"
+  ["timezone_type"]=>
+  int(3)
+  ["timezone"]=>
+  string(3) "UTC"
+}
+object(DateTime)#9 (3) {
+  ["date"]=>
+  string(26) "2018-12-31 00:00:00.000000"
+  ["timezone_type"]=>
+  int(3)
+  ["timezone"]=>
+  string(3) "UTC"
+}