Make this/next iiratio unsigned.
This patch addresses issue #79, which is a regression since commit 28de670c "Fix RD bug." If the coded error value is zero, the iiratio calculation effectively multiplies by 1000000 by the DOUBLE_DIVIDE_CHECK macro. This can result in a value larger than INT_MAX, giving a negative ratio. Since the error values are conceptually unsigned (though they're stored in a double) this patch makes the iiratio values unsigned, which allows the clamping to work as expected.