QProgressBar: prevent integer overflow

If `value` is INT_MAX and `lastPaintedValue` is -1 (the default), the
subtraction will overflow. Extend the arguments to 64 bits. The result
is only used for a double comparison anyhow.

Change-Id: Id4a5396c61da81932ac626b3102e583a74264df4
Pick-to: 6.9 6.8
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/widgets/qprogressbar.cpp

@@ -111,7 +111,7 @@ bool QProgressBarPrivate::repaintRequired() const
     if (value == lastPaintedValue)
         return false;
 
-    const int valueDifference = qAbs(value - lastPaintedValue);
+    const qint64 valueDifference = qAbs(qint64(value) - qint64(lastPaintedValue));
 
     if (value == minimum || value == maximum)
         return true;