Kill TCP connections if getsockname fails

Kill TCP connections if getsockname fails

In net-server.c:process_tcp_connection_read(), we don't expect
getsockname() to fail under ordinary circumstances, so instead of
passing a null local address to dispatch(), just error out.  Simplify
schpw.c:dispatch() by assuming a non-null local_saddr.

[ghudson@mit.edu: simplified schpw.c:dispatch(); rewrote commit
message]