Fix: silence 'non-consumed' message for non-started sessions

Fix: silence 'non-consumed' message for non-started sessions

Destroying a session with at least one enabled event and which has never
been started will currently result in an error message in the kernel log
about 'non-consumed data' for each of the per-cpu buffer. This happens
because a packet header is created in the buffer but never consumed if
the session is not started.

Add a check in the buffer cleanup code to avoid printing 'non-consumed
data' errors for buffers associated with a session taht was never
started.

Change-Id: I1358e1ae49d03544a961515b97b115a488434e27
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>