 dispatcher: Fix registration with length 16 IPv4 addresses (#3512)
 dispatcher: Fix registration with length 16 IPv4 addresses (#3512)

Make sure that an IPv4 is always 4 bytes long otherwise the parsing will fail.
Creating an IP from string via net.ParseIP/net.ResolveUDPAddr creates an IPv4 with a byte slice of length 16.
This was a problem for example for sciond.
Oncilla/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion