Drawing fix for overlay scrollbars on Mac.

A wrong check lead to a scrollbar with value 0 never been draw. This is
the fix for it. Instead is has to be checked for a length of 0.

Change-Id: I0c4e2f7e0014074e3c22554bcbea0ebfc3122952
Reviewed-by: Toby Tomkins <toby.tomkins@nokia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/widgets/styles/qmacstyle_mac.mm

@@ -5206,7 +5206,7 @@ void QMacStyle::drawComplexControl(ComplexControl cc, const QStyleOptionComplex
                     [scroller setFrame:NSMakeRect(0, 0, slider->rect.width(), height)];
                     CGContextTranslateCTM(cg, 0, value * totalHeight);
                 }
-                if (value > 0.0) {
+                if (length > 0.0) {
                     [scroller layout];
                     [scroller displayRectIgnoringOpacity:[scroller bounds]
                                                inContext:[NSGraphicsContext currentContext]];