Fix an issue that was reported in but unrelated to the main problem of

Fix an issue that was reported in but unrelated to the main problem of
SF bug 535905 (Evil Trashcan and GC interaction).

The SETLOCAL() macro should not DECREF the local variable in-place and
then store the new value; it should copy the old value to a temporary
value, then store the new value, and then DECREF the temporary value.
This is because it is possible that during the DECREF the frame is
accessed by other code (e.g. a __del__ method or gc.collect()) and the
variable would be pointing to already-freed memory.

BUGFIX CANDIDATE!