Fixed incorrect length handling in ngx_utf8_length(). Previously, ngx_utf8_decode() was called from ngx_utf8_length() with incorrect length, potentially resulting in out-of-bounds read when handling invalid UTF-8 strings. In practice out-of-bounds reads are not possible though, as autoindex, the only user of ngx_utf8_length(), provides null-terminated strings, and ngx_utf8_decode() anyway returns an errors when it sees a null in the middle of an UTF-8 sequence. Reported by Yunbin Liu.