dnsdist: Copy the DNS header before encrypting it in place

[rgacogne]

Short description

When DNSCrypt is enabled, we encrypt the answer in place, but we need to keep a copy of the DNS header of the original answer to be able to populate the responses ring buffer.

Fixes #5566.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[bjoe2k4]

Works for me. No Err#14 anymore. Thanks!

[rgacogne]

@bjoe2k4 Thank you for reporting, testing and confirming!