Don't rely on SA_LEN to check whether it's passed a null pointer (it

Don't rely on SA_LEN to check whether it's passed a null pointer (it
might be supplied by the OS, and the OS might supply a version that
doesn't check); instead, check for null pointers before doing anything
with the address at all.  This also means we don't supply a netmask if
the address is null; that makes sense, as a netmask makes no sense
without an address.