[CVE-2026-3591] sec: usr: Fix a stack use-after-return flaw in SIG(0) handling code

A stack use-after-return flaw in SIG(0) handling code could enable ACL
bypass and/or assertion failures in certain circumstances. This flaw has
been fixed.

ISC would like to thank Mcsky23 for bringing this vulnerability to our
attention.

Closes isc-projects/bind9#5754

Merge branch '5754-stack-use-after-free-sig0' into 'v9.21.20-release'

See merge request isc-private/bind9!920