detect missing message handler, and set a missing one

Author: kroening

src/langapi/language_util.cpp

@@ -95,6 +95,9 @@ exprt to_expr(
 {
   std::unique_ptr<languaget> p(get_language(ns, identifier));
 
+  null_message_handlert null_message_handler;
+  p->set_message_handler(null_message_handler);
+
   const symbolt &symbol=ns.lookup(identifier);
 
   exprt expr;

src/util/message.h

@@ -14,6 +14,7 @@ Author: Daniel Kroening, kroening@kroening.com
 #include <iosfwd>
 #include <sstream>
 
+#include "invariant.h"
 #include "source_location.h"
 
 class message_handlert
@@ -125,6 +126,7 @@ class messaget
 
   message_handlert &get_message_handler()
   {
+    INVARIANT(message_handler!=nullptr, "message handler is set");
     return *message_handler;
   }