Fix null pointer exceptions in CtfTmfTrace when location is null

Fix null pointer exceptions in CtfTmfTrace when location is null

This can happen when calling seekEvent(ITmfTimestamp) in TmfTrace with a
time stamp that is past the end of the trace.

Change-Id: I5656b196164925f8179768f9958be1fc1b040550
Reviewed-on: https://git.eclipse.org/r/8801
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>