Fix leak in DateTimeImmutable::modify()

Author: nikic

ext/date/php_date.c

@@ -3297,6 +3297,7 @@ PHP_METHOD(DateTimeImmutable, modify)
 
 	date_clone_immutable(object, &new_object);
 	if (!php_date_modify(&new_object, modify, modify_len)) {
+		zval_ptr_dtor(&new_object);
 		RETURN_FALSE;
 	}
 

ext/date/tests/DateTimeImmutable_modify_invalid_format.phpt

@@ -0,0 +1,12 @@
+--TEST--
+DateTimeImmutable::modify() with invalid format
+--FILE--
+<?php
+
+$datetime = new DateTimeImmutable;
+var_dump($datetime->modify(''));
+
+?>
+--EXPECTF--
+Warning: DateTimeImmutable::modify(): Failed to parse time string () at position 0 ( in %s on line %d
+bool(false)