Fix: wrong check on session started on stop command

This is problematic for application that lives longer than the tracing
session so the make check unfortunately did not catch this problem since
we either kill the applications before the stop or wait for them to die.

I will quote a colleague of mine on IRC after discovering this:
14:14 < cbab> moar tests!

:)

Signed-off-by: David Goulet <dgoulet@efficios.com>