Crash in 'katana_stringify_value_list'.

[vs-savchenko]

Here (parser.c:1635):

        const char* value_str = katana_stringify_value(parser, value);
        katana_string_append_characters(parser, value_str, buffer);
        katana_parser_deallocate(parser, (void*) value_str);
        value_str = NULL;

'katana_stringify_value' can actually return NULL, if value is empty KATANA_VALUE_PARSER_LIST.
After that katana_string_append_characters will call strlen(NULL).