Fix casting away const in exception_listt

`find` always returns a const reference. Therefore we need to use `add`,
to get a mutable reference instead. Otherwise the `.get_sub()` may yield
the global empty sub instead of a freshly allocated one.

Author: thomasspriggs

src/util/std_code.h

@@ -1858,7 +1858,7 @@ class code_push_catcht:public codet
   }
 
   exception_listt &exception_list() {
-    return (exception_listt &)find(ID_exception_list).get_sub();
+    return (exception_listt &)add(ID_exception_list).get_sub();
   }
 
   const exception_listt &exception_list() const {