dnsdist: Prevent a dangling DOHUnit pointer when send() failed

[rgacogne]

Short description

In the quite unlikely case where sending a DoH query toward the backend over the UDP socket failed, we could have left a dangling pointer to the DOHUnit struct, possibly leading to a crash when the IDState was cleaned up later.

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)