miniupnpd/natpmp.c: skip port zero when finding free eport

When skipping ports that are in use or not allowed, the existing
NAT-PMP code will consider port zero as a candidate eport after
wraparound occurs. Since this is not a legal port, we skip over it.

port zero as an eport value.
Merge pull request #57 from razzfazz/natpmp_avoid_port_zero

miniupnpd/natpmp.c: skip port zero when finding free eport