fix: redraw only if the buffer is valid

[Biggybi]

Hi, and thank you for a great plugin.

This PR addresses an issue where Alpha attempts to redraw when the buffer is invalid, causing the WinResized autocmd to trigger in a loop.

   Error  12:10:54 AM msg_show.lua_error Error detected while processing WinResized Autocommands for "*":

It seems to occur if an autocmd on VimEnter opens buffers that we edit/resize.

I believe this check is reasonable, but I understand if you feel it should not be Alpha's responsibility.

Repro:

I encountered this issue using this script.

• open neovim, and create a file through Alpha, add text (autoscratch creates a new file)
• exit and reopen neovim (autoscratch opens the file)
• edit the file or resize the window

Related: #282.