Fix QTimer with negative interval in QWidgetLineControl::setCursorBlinkPeriod

When keyboard blink is disabled, msec will be negative (-2 in my case).
In that case if (msec) will evaluate to true and create a QTimer
with a negative interval. Fix is to check for msec > 0.

Change-Id: I5c8d82c3cdaf26a961c986f4164805aacfe3ae37
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>