Fix: data reception is called event when only FDS are expected for reception

Fix: data reception is called event when only FDS are expected for reception

In non blocking mode, the FDs reception code can hit EAGAIN. In such
cases, skip the data reception completely when performing the reception.

The same behavior could have been implemented with a noop at the
`lttcomm_recv_unix_sock_non_block` level when the passed `len` is equal
to 0. The handling of it at the notification thread reception level was
chosen to be explicit with regards to the expected behaviour and not
rely on implicit functionality of `lttcomm_recv_unix_sock_non_block`.

While there, augment assertions for code in unix.c

Change-Id: I0fa6df1c9630caa6e877a39372bdfd3d3a01b648
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>