fix: usr: Do not assert on synthrecord reverse mode with huge prefix

When using the `synthrecord` plugin in reverse mode, if a very long
prefix is configured by the operator such that there is no room to fit
the reversed IP address into a DNS name, `named` could assert. This has
now been fixed. In such situations, an error is logged so the operator
is aware of the problem, and `NXDOMAIN` is answered.

Closes #6115

Merge branch '6115-synthrecord-prefix' into 'main'

See merge request isc-projects/bind9!12173