[9.20] fix: usr: Valid TSIG signatures with invalid time cause crash

An assertion failure triggers when the TSIG has valid cryptographic signature, but the time is invalid. This can happen when the times between the primary and secondary servers are not synchronised.

Closes #4811

Backport of MR !9234

Merge branch 'backport-4811-fix-isc_buffer_putuint48-buffer-size-requirement-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9245