prelude: don't build string objet for NULL string

prelude_string_set_ref don't like when it is called with a NULL
parameter. This patch adds check for NULL value. This is formally
good as there is no use of a NULL description.

src/alert-prelude.c

@@ -249,11 +249,13 @@ static int EventToImpact(PacketAlert *pa, Packet *p, idmef_alert_t *alert)
         idmef_assessment_set_action(assessment, action, 0);
     }
 
+    if (pa->s->class_msg) {
         ret = idmef_impact_new_description(impact, &str);
         if ( ret < 0 )
             SCReturnInt(ret);
 
         prelude_string_set_ref(str, pa->s->class_msg);
+    }
 
     SCReturnInt(0);
 }
@@ -693,11 +695,13 @@ TmEcode AlertPrelude (ThreadVars *tv, Packet *p, void *data, PacketQueue *pq, Pa
     if ( ret < 0 )
         goto err;
 
+    if (pa->s->msg) {
         ret = idmef_classification_new_text(class, &str);
         if ( ret < 0 )
             goto err;
 
         prelude_string_set_ref(str, pa->s->msg);
+    }
 
     ret = EventToImpact(pa, p, alert);
     if ( ret < 0 )