channel: Move mch_exit(0) call to a non-deferred event handler

Since `mch_exit` will re-enter event_poll, it is necessary to call it outside
libuv event loop.

src/nvim/msgpack_rpc/channel.c

@@ -600,11 +600,16 @@ static void close_channel(Channel *channel)
     if (handle) {
       uv_close(handle, close_cb);
     } else {
-      mch_exit(0);
+      event_push((Event) { .handler = on_stdio_close }, false);
     }
   }
 }
 
+static void on_stdio_close(Event e)
+{
+  mch_exit(0);
+}
+
 static void free_channel(Channel *channel)
 {
   pmap_del(uint64_t)(channels, channel->id);