The ByteBuffer get methods use a java comparison between the offset and the
upper bound.  This is a signed comparison and doesn't work if we have mapped the
ByteBuffer onto a 64-bit memory space like on AMD64.  A special unsigned
comparison routine is required.  In Java, this could be:

boolean uLongLessThan (a, b)
{
   if (a >=0 && b >= 0 || a < 0 && b < 0)
        return a < b;
   else
        return b < 0;
}

Alternatively, the comparison could be pushed down to cni code and done in C
where unsigned comparisons can be made.