QQuickTimeLine: add missing time checks

The same check is done in QQuickTimeLine::pause(), move() and moveBy(),
where the time is passed as an argument. QQuickTimeLine::accel() and
accelDistance() calculate the time based on velocity and acceleration
or distance. With tiny values this calculation can result to a negative
time due to integer overflow.

Task-number: QTBUG-35046
Change-Id: I000e73d3f787375946e8f87a5d24153ae919bc8d
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Merge pull request #4 from mbrasser-jolla/timeline-assert

QQuickTimeLine: add missing time checks