fix(rpc): ignore redraw events when not in UI client (#21892)

Otherwise it will crash.

src/nvim/msgpack_rpc/channel.c

@@ -247,11 +247,13 @@ static void parse_msgpack(Channel *channel)
   Unpacker *p = channel->rpc.unpacker;
   while (unpacker_advance(p)) {
     if (p->type == kMessageTypeRedrawEvent) {
+      if (ui_client_channel_id) {
         if (p->grid_line_event) {
           ui_client_event_raw_line(p->grid_line_event);
         } else if (p->ui_handler.fn != NULL && p->result.type == kObjectTypeArray) {
           p->ui_handler.fn(p->result.data.array);
         }
+      }
       arena_mem_free(arena_finish(&p->arena));
     } else if (p->type == kMessageTypeResponse) {
       ChannelCallFrame *frame = kv_last(channel->rpc.call_stack);

test/functional/api/server_requests_spec.lua

@@ -337,6 +337,21 @@ describe('server -> client', function()
       eq('localhost:', string.sub(address,1,10))
       connect_test(server, 'tcp', address)
     end)
+
+    it('does not crash on receiving UI events', function()
+      local server = spawn(nvim_argv)
+      set_session(server)
+      local address = funcs.serverlist()[1]
+      local client = spawn(nvim_argv, false, nil, true)
+      set_session(client)
+
+      local id = funcs.sockconnect('pipe', address, {rpc=true})
+      funcs.rpcrequest(id, 'nvim_ui_attach', 80, 24, {})
+      assert_alive()
+
+      server:close()
+      client:close()
+    end)
   end)
 
   describe('connecting to its own pipe address', function()