This fixes all problems from the article "Checking OpenCV with PVS-Studio"
<http://www.viva64.com/en/b/0191/> that are not already fixed and are
not in 3rdparty or the legacy module.

The problems fixed are two instances of useless code and one instance
of unspecified behavior (right-shifting a negative number).

Consistent with the test made in cv::boxFilter, it adjusts the kernel size to the source size only if the border is not BORDER_CONSTANT and if BORDER_ISOLATED is set. Otherwise, the source has to be considered possibly in a larger image (i.e. the source being a ROI) in witch the kernel should apply.