dnsdist: Prevent an underflow of the TCP d_queued counter

[rgacogne]

Short description

By incrementing it before writing to the pipe, and decrementing it in case of an error, we prevent a very possible underflow from occurring if the reader manages to decrement before we can return from write and increment it.
Fixes #12357

Checklist

I have:

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

[Berndinox]

Waiting for this, thanks for fixing!