fix: usr: Prevent resolver crashes while processing DNS over TCP

Recursive resolvers could terminate with an assertion failure
while processing DNS responses over TCP under sustained traffic.
The failure was observed on resolvers configured globally with
forward only; the same transport path is also used by iterative
resolution.  This has been fixed.

Merge branch 'ondrej/prevent-resolver-crash-over-tcp' into 'main'

See merge request isc-projects/bind9!12536