avoid access to exprt::op1

Author: Daniel Kroening

src/goto-symex/symex_assign.cpp

@@ -417,7 +417,7 @@ void goto_symext::symex_assign_struct_member(
   //   a'==a WITH [c:=e]
 
   with_exprt new_rhs(lhs_struct, exprt(ID_member_name), rhs);
-  new_rhs.op1().set(ID_component_name, component_name);
+  new_rhs.where().set(ID_component_name, component_name);
 
   exprt new_full_lhs=add_to_lhs(full_lhs, lhs);