Fix a very subtle bug in the initialisation of VSD domain objects

The default state of a domain on construction should be bottom.
This is documented in the ai_domaint interface.
As the variable_sensitivity_domaint delegates the handling of bottom
to the abstract_environment, it must be initialised to bottom in the
default case.

Author: martin

src/analyses/variable-sensitivity/abstract_enviroment.cpp

@@ -803,6 +803,6 @@ abstract_environmentt::gather_statistics(const namespacet &ns) const
   return statistics;
 }
 
-abstract_environmentt::abstract_environmentt() : bottom(false)
+abstract_environmentt::abstract_environmentt() : bottom(true)
 {
 }