The memmove() call should be using the index argument to determine the
number of bytes to copy. To be consistent with the rest of the code,
we should also not evaluate the index parameter multiple
times. Calling this with rand() % arr.size would otherwise generally
segfault.

Finally, we want to avoid using "index" as an identifier so as to not
shadow index(3) in the C library.

Signed-off-by: Damien Grassart <damien@grassart.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>