Fix release INVITE server request lead to core dump at worng order 200OK.

Author: fanpengy

libsofia-sip-ua/nua/nua_server.c

@@ -425,8 +425,13 @@ nua_stack_respond(nua_t *nua, nua_handle_t *nh,
     if (request && sr->sr_request.msg == request)
       break;
     /* nua_respond() to INVITE can be used without NUTAG_WITH() */
-    if (!t && sr->sr_method == sip_method_invite)
+    if (!t && sr->sr_method == sip_method_invite) {
       break;
+    } else if(!t && (sr->sr_method == sip_method_prack || sr->sr_method == sip_method_update)) {
+      nua_stack_event(nua, nh, NULL, nua_i_error,
+		    500, "Server Internal Error", NULL);
+      return;
+    }
   }
 
   if (sr == NULL) {