snet: fix panic in Listen (#1806)
snet: fix panic in Listen (#1806)

Add more checks to avoid the panic and also provide a default value if
the address has no L4 information. In such case, the default value
would be UDP port 0, meaning that the application wants a random port.

Fixes #1803
Oncilla/scion,netsec-ethz/scion,klausman/scion,klausman/scion,klausman/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion,Oncilla/scion,klausman/scion,klausman/scion