ctf: fix npe in size() if trace is not inited

ctf: fix npe in size() if trace is not inited

This is a race condition that can appear when bulk openning 5-6
ctf traces at onces.

Change-Id: I8f64d373674e6b1b925a43e607763c518f866ccb
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/77361
Reviewed-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>