Fix: remove assert on fd in the read/write layer

It is possible that an invalid fd is passed to read or write. This can
happen for instance if the endpoint of the transport (ex: relayd) dies
out while actively trying to send data. It is OK to let an invalid fd
where the syscall will return the right value along with errno being
populated with the corresponding code.

Acked-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>