Don't try to relay packets to unreachable nodes.

It is not unusual for tinc to receive SPTPS packets to be relayed to
nodes that just became unreachable, due to state propagation delays in
the metagraph.

Unfortunately, the current code doesn't handle that situation correctly,
and still tries to relay the packet to the unreachable node. This
typically ends up segfaulting.

This commit fixes the issue by checking for reachability before relaying
the packet.