Check validity of constraints' lvalues during PolicyCheckPartial().

The same check existed in the parser so I removed it. Validity of constraints
is now checked in ConstraintCheckSyntax(), copied from ConstraintGetSyntax()
which now expects syntax is already checked.

Also fixed a segfault, so changed the relevant acceptance test to not expect
crash, but just failure.
Merge pull request #397 from jimis/bug2230

Check validity of constraints' lvalues during PolicyCheckPartial().