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.

This 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: I6678789105e359fa26c99409f449cad0ba3bd0ae
(cherry picked from qtstyleplugins/8ff2ac6035fb1d01f5c0054ba14afb949410e3a7)
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>