Avoid undefined arithmetic on nullptr in buffer_offset function

Avoid undefined arithmetic on nullptr in buffer_offset function

Not quite sure why it was done that way. Maybe to ensure we work with byte-pointers of correct size or something?
Anyway, this doesn't seem to be defined in c++, so let's try working with a cast instead.
Just something cppcheck tool complained about.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6447 dfc29bdd-3216-0410-991c-e03cc46cb475