dnsdist: Fix invalid `substr()` use in the DNS overlay parser

dnsdist: Fix invalid `substr()` use in the DNS overlay parser

`std::basic_string_view<CharT,Traits>::substr`'s second parameter is a length,
not an iterator or a position, so the existing code was misusing it and
creating a view that potentially expanded outside of the packet.
However currently the view is never used to read more than
`record.d_contentOffset` (we are passing it immediately to `makeComboAddressFromRaw`
with `record.d_contentLength` as the length) and `record.d_contentOffset`
has been validated right before to be either `4` or `16`, so
there is no out-of-bounds read.
This issue has been introduced in b6f9a21db93ee25ec665dc5f65e87eb7adebd102 and
is not included in any stable release, so no need to backport
the fix.

Reported by Nyaz360 in YWH-PGM6095-85, thanks a lot!

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>