s4:sam.py - assign valid values when performing the special-attributes constraint...

s4:sam.py - assign valid values when performing the special-attributes constraint checks

The problem is that s4 per construction does the checks in a different order. It
first checks for validity (pre-operation trigger in samldb LDB module) and then
for the schema (post-operation trigger in objectclass_attrs LDB module).
constraints (post-operation trigger