184.108.40.206: room: Fix map-stack-references for stack-grows-upward systems.
* SAP+ is (function (system-area-pointer integer) (values
* SAP- is (function (system-area-pointer system-area-pointer) (values
* When negating the delta in a SAP+ operation, don't use SAP-.
Instead, negate the offset. Otherwise, you get a WARNING for passing an
integer where a system-area-pointer is expected.