fix constructor of field_address_exprt

This fixes the constructor of field_address_exprt.

Author: kroening

src/util/pointer_expr.cpp

@@ -138,9 +138,9 @@ field_address_exprt::field_address_exprt(
   pointer_typet _type)
   : unary_exprt(ID_field_address, std::move(compound_ptr), std::move(_type))
 {
-  const auto &compound_ptr_type = compound_ptr.type();
-  PRECONDITION(compound_ptr_type.id() == ID_pointer);
-  const auto &compound_type = to_pointer_type(compound_ptr_type).base_type();
+  const auto &base_type = field_address_exprt::base().type();
+  PRECONDITION(base_type.id() == ID_pointer);
+  const auto &compound_type = to_pointer_type(base_type).base_type();
   PRECONDITION(
     compound_type.id() == ID_struct || compound_type.id() == ID_struct_tag ||
     compound_type.id() == ID_union || compound_type.id() == ID_union_tag);