Cleanlooks style: Fix floating point exception

In QCleanlooksStyle::drawControl, if indeterminate == true and
rect.width() == 4, we will end up with slideWidth of zero, and
take a mod by zero when calculating the value of step.

Identical code in Qt4 Cleanlooks causes a crash in Quassel 0.10;
see https://bugs.gentoo.org/show_bug.cgi?id=507124

Instead, calculate slideWidth based on max(width, minWidth) where
minWidth was already set as 4, ensuring that slideWidth >= 2.

Change-Id: Id3e39437665be326358f68c48eaf1249b6880c4a
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>