[9.20] fix: nil: correct the DbC assertions in message.c

the comments for some calls in the dns_message API specified
requirements which were not actually enforced in the functions.
in most cases, this has now been corrected by adding the missing
REQUIREs. in one case, the comment was incorrect and has been
revised.

Backport of MR !10466

Merge branch 'backport-each-fix-message-requires-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10484