Fix: Unchecked session pointer when destroying a connection in relayd

An unknown command currently crashes the relay daemon since
destroy_connection calls destroy_session without checking whether or not
a session is associated with the connection.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>