Bluetooth: Host: Add "assert non-null" to bt_conn_ref

Clearly `conn` must be non-null since it's dereferenced without checks.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>

subsys/bluetooth/host/conn.c

@@ -1120,6 +1120,8 @@ struct bt_conn *bt_conn_ref(struct bt_conn *conn)
 {
 	atomic_val_t old;
 
+	__ASSERT_NO_MSG(conn);
+
 	/* Reference counter must be checked to avoid incrementing ref from
 	 * zero, then we should return NULL instead.
 	 * Loop on clear-and-set in case someone has modified the reference