out the for loop at the end intended to zero out new items wasn't
doing anything, because sv->ob_size was already equal to newsize.  The
fix slightly refactors the function, introducing a variable oldsize
and doing away with sizediff (which was used only once), and using
oldsize and newsize consistently.  I also added comments explaining
what the two for loops do.  (Looking at the CVS annotation of this
function, it's no miracle a bug crept in -- this has been patched by
many different folks! :-)