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.