Validate config is initialised when a member_offset_expr is constructed

This will save time hunting for the cause of problems when unit tests don't initialise the config.

Author: NathanJPhillips

src/util/pointer_offset_size.cpp

@@ -241,6 +241,7 @@ optionalt<exprt> member_offset_expr(
   const irep_idt &member,
   const namespacet &ns)
 {
+  PRECONDITION(size_type().get_width() != 0);
   exprt result=from_integer(0, size_type());
   std::size_t bit_field_bits=0;