Fix issue nikic#36

The emulation of PHP 7.0 list() structure did not account for
skipped elements.

Author: nikic

ast.c

@@ -241,7 +241,7 @@ static inline zend_bool ast_array_is_list(zend_ast *ast) {
 	}
 
 	for (i = 0; i < list->children; i++) {
-		if (list->child[i]->child[1] != NULL || list->child[i]->attr) {
+		if (list->child[i] && (list->child[i]->child[1] != NULL || list->child[i]->attr)) {
 			return 0;
 		}
 	}
@@ -400,7 +400,7 @@ static void ast_fill_children_ht(HashTable *ht, zend_ast *ast, zend_long version
 			/* Skip docComment child -- It's handled separately */
 			continue;
 #if PHP_VERSION_ID >= 70100
-		} else if (ast->kind == ZEND_AST_LIST) {
+		} else if (ast->kind == ZEND_AST_LIST && child != NULL) {
 			/* Emulate simple variable list */
 			ast_to_zval(&child_zv, child->child[0], version);
 #else

tests/array_destructuring_old.phpt

@@ -8,6 +8,7 @@ require __DIR__ . '/../util.php';
 $code = <<<'PHP'
 <?php
 list($a, $b) = $x;
+list(, $b) = $x;
 PHP;
 
 echo ast_dump(ast\parse_code($code, $version=30)), "\n";
@@ -24,6 +25,13 @@ AST_STMT_LIST
                 name: "b"
         expr: AST_VAR
             name: "x"
+    1: AST_ASSIGN
+        var: AST_LIST
+            0: null
+            1: AST_VAR
+                name: "b"
+        expr: AST_VAR
+            name: "x"
 AST_STMT_LIST
     0: AST_ASSIGN
         var: AST_ARRAY
@@ -40,3 +48,14 @@ AST_STMT_LIST
                 key: null
         expr: AST_VAR
             name: "x"
+    1: AST_ASSIGN
+        var: AST_ARRAY
+            flags: ARRAY_SYNTAX_LIST (1)
+            0: null
+            1: AST_ARRAY_ELEM
+                flags: 0
+                value: AST_VAR
+                    name: "b"
+                key: null
+        expr: AST_VAR
+            name: "x"