Add validation for reference_typet

A reference type without a width is still a reference type, it's just an invalid one.
Rather than the cast failing, a DATA_INVARIANT should be put on this property.

Author: NathanJPhillips

src/util/std_types.h

@@ -1565,8 +1565,13 @@ class reference_typet:public pointer_typet
 template <>
 inline bool can_cast_type<reference_typet>(const typet &type)
 {
-  return can_cast_type<pointer_typet>(type) && type.get_bool(ID_C_reference) &&
-         !type.get(ID_width).empty();
+  return can_cast_type<pointer_typet>(type) && type.get_bool(ID_C_reference);
+}
+
+inline void validate_type(const reference_typet &type)
+{
+  DATA_INVARIANT(!type.get(ID_width).empty(), "reference must have width");
+  DATA_INVARIANT(type.get_width() > 0, "reference must have non-zero width");
 }
 
 /// \brief Cast a typet to a \ref reference_typet