tmf: Fix integer overflow in time graph calculations

tmf: Fix integer overflow in time graph calculations

An x-coordinate very far to the right could overflow to be very far to
the left. This could prevent states and/or their labels to be drawn when
zoomed-in very close.

Change-Id: I6e006d366dfe427a9ac7df1a8f534fe95c230c3d
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/79565
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>