[9.18] fix: usr: Fix a memory leak issue in the catalog zones

The :iscman:`named` process could leak small amounts of memory
when processing a catalog zone entry which had defined custom
primary servers with TSIG keys using both the regular ``primaries``
custom property syntax and the legacy alternative syntax (``masters``)
at the same time. This has been fixed.

Closes #5943

Backport of MR !11951

Merge branch 'backport-5943-catz-primaries-tsig-key-name-leak-fix-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11974