Work around a GCC bug that causes inet_checksum() to give wrong results.

Work around a GCC bug that causes inet_checksum() to give wrong results.

Valgrind reports the following bug:

==24877== Conditional jump or move depends on uninitialised value(s)
==24877==    at 0x12283E: inet_checksum (route.c:80)
==24877==    by 0x12283E: route_ipv6_unreachable (route.c:315)
==24877==    by 0x1236AC: route_ipv6 (route.c:751)
==24877==    by 0x1236AC: route (route.c:1160)
==24877==    by 0x113DE0: receive_tcppacket (net_packet.c:493)
==24877==    by 0x1119D4: receive_meta (meta.c:315)
==24877==    by 0x113288: handle_meta_connection_data (net.c:287)
==24877==    by 0x11A091: handle_meta_io (net_socket.c:491)
==24877==    by 0x10FB0C: event_loop (event.c:370)
==24877==    by 0x11362E: main_loop (net.c:489)
==24877==    by 0x10CACA: main (tincd.c:551)

Clearing the variable pseudo in route_ipv6_unreachable removes this error,
but the resulting checksum is still bad. If one instead adds a dummy
write that depends on checksum, the error goes away and the checksum is
correct.