dnsmasq: refuse to add empty DHCP range

Use ipcalc's return value to react to invalid range specifications.
By simply ignoring the range instead of aborting with an error code,
dnsmasq should still start when there's an error (best effort).
Aborting the config generation or working with invalid range specs leaves
dnsmasq crash-looping which is the right thing to do concerning that
particular interface but it also hinders DHCP service on other interfaces
and DNS on the router itself.

Signed-off-by: Leon M. George <leon@georgemail.eu>