dns plugin: Don't pass a NULL pointer to `pcap_open_live'.

Although the documentation states clearly, that passing a NULL pointer
as device is okay and handled like "any", doing so will crash the daemon
on some systems, most notably *BSDs but Linux users have reported this
behavior, too.

This patch passes "any" when the pointer it NULL, which reportedly
resulted in a different behavior, but still crashing the daemon. We'll
keep trying ;)