Fix time scaling problem in Histogram View

Fix time scaling problem in Histogram View

If scale of TmfTimestamps of a trace is not nano-seconds
the start and end time of a trace have to be scaled when comparing
with entered time values.

Additionally, a NPE was fixed that was happening when clearing
the histogram while executing asyncExec() to update the display

Change-Id: I39dd3ed3e375ee520678f1de9cb45ee5fccf77d5
Reviewed-on: https://git.eclipse.org/r/8985
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Hudson CI