[9.20] [CVE-2025-40775] sec: usr: Prevent assertion when processing TSIG algorithm

DNS messages that included a Transaction Signature (TSIG) containing an
invalid value in the algorithm field caused :iscman:`named` to crash
with an assertion failure. This has been fixed.  :cve:`2025-40775`

Backport of !793

See isc-projects/bind9#5300

Merge branch '5300-confidential-tsig-unknown-alg-bind-9.20' into 'v9.20.9-release'

See merge request isc-private/bind9!795