fix: usr: Fix a catalog zones issue when a member zone could fail to load

A catalog zone's member zone could fail to load in some rare cases, when
the internally generated zone configuration string was exceeding 512
bytes. That condition only was not enough for the issue to arise, but it
was a necessary condition. This could happen, for example, if the catalog
zone's default primary servers list contained a large number of items.
This has been fixed.

Closes #5658

Merge branch '5658-dns_catz_generate_zonecfg-bug-fix' into 'main'

See merge request isc-projects/bind9!11281