Fix: lttng: Destroying session message repeated during destruction

Fix: lttng: Destroying session message repeated during destruction

Observed Issue
==============

The `Destroying session X` is repeated indifinitely whenever
the data pending phase lasts more than one iteration.

```
$ lttng destroy
Destroying session eloi_turcotte.Destroying session
eloi_turcotte.Destroying session eloi_turcotte.Destroying session
eloi_turcotte.D
```

Cause
=====

Missing check that the message has been printed.

Solution
========

Use the same check as is done later for
lttng_destruction_handle_wait_for_completion().

Known drawbacks
===============

None.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6cd29d917925644a4994c515b4177bbd05ffa98e