Don't allow list markers to be indented >= 4 spaces.

See commonmark/commonmark-spec#497.

Author: jgm

src/blocks.c

@@ -945,6 +945,7 @@ static void open_new_blocks(cmark_parser *parser, cmark_node **container,
                              parser->first_nonspace + 1);
       S_advance_offset(parser, input, input->len - 1 - parser->offset, false);
     } else if ((!indented || cont_type == CMARK_NODE_LIST) &&
+	       parser->indent < 4 &&
                (matched = parse_list_marker(
                     parser->mem, input, parser->first_nonspace,
                     (*container)->type == CMARK_NODE_PARAGRAPH, &data))) {