Corrected failing test on 32-bit systems

Corrected failing test on 32-bit systems

When size_t is not uint64_t, converting "negative" size_t integers to
uint64_t yields nonsensical results.  That is, the following isn't
portable:

    size_t   x = 42;
    uint64_t y = -i;

Because y might be missing the high order bits if size_t is smaller than
uint64_t. Instead, we want to convert to a large sized integer *before*
we negate it:

    size_t   x = 42;
    uint64_t y = -(uint64_t)i;