Fix: create_kernel_session asserts on failure

create_kernel_session() will call trace_kernel_destroy_session()
on failure to create a kernel session (e.g. modules failed to load).

This can be reproduced by enabling kernel events on a session after
the session daemon has failed to load the LTTng kernel modules.

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