spurious closed channel errors when killing the game buffer

[seandewar]

May (rarely) get errors like E900: Invalid channel id if a scheduled event that writes to the terminal channel happens while killing the game buffer.

Presumably it's because the terminal is closed before buf_freeall autocommands (see close_buffer in Nvim), which should've :close()'d the game instance before that point. Actually, not what's happening here, as the event is being invoked via the main loop; still that scenario might be possible if there's another autocommand that processes non-fast events before ours...

In a perfect world, the terminal would only be freed after we're sure the buffer is going to be deleted (after buf_freeall autocommands), so it's a bit of an Nvim L, but might be nice to workaround that.