dnsmasq: Fix hosts file format when MAC address is not specified

dnsmasq: Fix hosts file format when MAC address is not specified

An entry like this in /etc/config/dhcp:

    config 'host'
        option 'name' 'pc2'
        option 'ip' '192.168.100.56'
        option 'dns' '1'

results in a /tmp/hosts/dhcp entry that looks like this:

    192.168.100.56 .lan

Obviously it should say "pc2.lan".

This happens because $name is set to "" in order to support the MAC-less
syntax: "--dhcp-host=lap,192.168.0.199".  Fix this by reordering the
operations.  Also, refuse to add a DNS entry if the hostname or IP is
missing.

Fixes #17683

Reported-by: Kostas Papadopoulos <kpapad75@travelguide.gr>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Backport of r42319

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42389 3c298f89-4303-0410-b956-a3cf2f4a3e73