tmf: Fix zero timestamp being included in all time ranges

tmf: Fix zero timestamp being included in all time ranges

For some obscure unknown reason, if a timestamp's value was equal
to 0, the TmfTimeRange.contains() method would always consider that
timestamp to be included in the time range, which is obviously wrong.

Change-Id: I61afea2eb05ede61b02dbaaed388e1a49c041fa2
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/17980
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>