dnsdist: Detect and dismiss truncated UDP responses from a backend

dnsdist: Detect and dismiss truncated UDP responses from a backend

Until now we would not have detected if the response was larger than
our buffer (4096 bytes or larger in some cases), which could have
led to parsing errors or even forwarding a corrupted response.

(cherry picked from commit 17a0b06ee3cfa43bb9ac4a0cd5186fbc42e090a5)