Fix off-by-one error in ArrayAllocatingPolicy::releaseResource.

A pointer "r" is contained in a bucket when the condition

    firstItem > r || r > firstItem + BucketSize

is fulfilled. I introduced an off-by-one error when negating this
expression. This lead to the situation where the last-plus-one item
of a bucket was said to belong to a bucket, while it should not.

Change-Id: I000e774f55ba4575348058f9efd70e606fa1baa4
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>