Fix incorrect access of AST_UNPACK

list_is_keyed() did not take into account that there may be
AST_UNPACK elements. These would error lateron anyway, but still
produce an invalid access here.

Zend/tests/array_unpack/in_destructuring_2.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Spread operator is not supported in destructuring assignments (only spread)
+--FILE--
+<?php
+
+[...$x] = [1, 2, 3];
+
+?>
+--EXPECTF--
+Fatal error: Spread operator is not supported in assignments in %s on line %d

Zend/zend_compile.c

@@ -3006,8 +3006,9 @@ static bool zend_propagate_list_refs(zend_ast *ast) { /* {{{ */
 static bool list_is_keyed(zend_ast_list *list)
 {
 	for (uint32_t i = 0; i < list->children; i++) {
-		if (list->child[i]) {
-			return list->child[i]->child[1] != NULL;
+		zend_ast *child = list->child[i];
+		if (child) {
+			return child->kind == ZEND_AST_ARRAY_ELEM && child->child[1] != NULL;
 		}
 	}
 	return false;