Prelude: fix test always returning true

Fix wrong logic in test for error handling code.

Signed-off-by: Pierre Chifflier <chifflier@wzdftpd.net>

src/alert-prelude.c

@@ -329,7 +329,7 @@ static int EventToSourceTarget(Packet *p, idmef_alert_t *alert)
         SCReturnInt(ret);
 
     ret = idmef_target_new_service(target, &service);
-    if ( ! ret < 0 )
+    if ( ret < 0 )
         SCReturnInt(ret);
 
     if ( p->tcph || p->udph )