Fix: lttng: initialize sessions pointer to NULL

lttng_list_sessions does not set the passed pointer to NULL on empty
return. This leads to a deallocation of an invalid pointer (segfault).

For returns of size 0, the value of the passed argument should be
considered "undefined".

Refactor error handling a bit by removing the "error" jump. Always
call free on the 'sessions' object.

Fixes #1205

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