In [RT #43367], the BIND 9 changed the strictness of address / prefix
length checks:
Check prefixes in acls to make sure the address and
prefix lengths are consistent. Warn only in
BIND 9.11 and earlier.
Unfortunately, a regression slipped in and the check was made an error
also in the BIND 9.11. This commit fixes the regression, but turning
the error into a warning.