Fix: sessiond: missing goto in error handler

The trace_ust inclusion set add/remove methods do not jump to the
end label after checking the `tracker` variable. This can result
in a NULL pointer dereference when an invalid process attribute
is specified.

The same problem appears in save_process_attr_trackers() and
process_attr_value_from_comm().

The missing jump (goto) is added in all cases.

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I473e008e5330a4c3820c8ab7c57ce4f2961e79b2