Fix for wrong icons buffer initialization happens for cards
without NPOT support. Was wrong stride used for memcpy leading
to wrong memory writes in def_internal_icon.

It's a regression since matcap commit.

Should fix the following reports:
- #33993: Crash on Blender startup (Vista x32)
- #33996: Latest build crashes on win xp

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54150 954f8c5b-7b00-dc11-b283-0030488c597c