tmf: annotate TmfContext#location as nullable

tmf: annotate TmfContext#location as nullable

This fixes a potential race condition if the context is changed
after a null check but before a dereference. In the current
implementation, it is extremely unlikely to happen, but this
may save later investigation time.

Change-Id: I9083d16fd8e3728fe829583f4de78384109a91e6
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/82325
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Hudson CI