Fix host_name_lookup (Close 2747)

Thanks to Nico R for providing a reproducing configuration.

        host_lookup             = *
        message_size_limit      = ${if def:sender_host_name {32M}{32M}}
        acl_smtp_connect        = acl_smtp_connect
        acl_smtp_rcpt           = acl_smtp_rcpt

        begin acl
          acl_smtp_connect:
                warn ratelimit = 256 / 1m / per_conn
                accept

          acl_smtp_rcpt:
                accept hosts = 127.0.0.*

        begin routers
        null:
          driver          = accept
          transport       = null

        begin transports
        null:
          driver          = appendfile
          file            = /dev/null

Tested with

        swaks -f mailbox@example.org -t mailbox@example.org --pipe 'exim -bh 127.0.0.1 -C /opt/exim/etc/exim-bug.conf'

The IP must have a PTR to "localhost." to reproduce it.

(cherry picked from commit 20812729e3e47a193a21d326ecd036d67a8b2724)