Merge typo, forgot to fix the access type.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4617 dfc29bdd-3216-0410-991c-e03cc46cb475
Fix memory access in d3dx mipmap generation. Thanks to hellflip for the hint.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4618 dfc29bdd-3216-0410-991c-e03cc46cb475