runtime(optwin): Fix E94 when searching for the option-window

Problem:  When the parameter debug=msg is set and the command :option is
          entered, error E94 will be displayed.
Solution: Add a check for the existence of the buffer before getting the
          buffer number “option-window”.

Reproduce:

vim --clean -c "set debug=msg" -c "option"

    Error detected while processing command line..script D:\Programs\Vim\vim91\optwin.vim:
    line 9: E94: No matching buffer for option-window

closes: #17927

Signed-off-by: RestorerZ <restorer@mail2k.ru>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: RestorerZ

runtime/optwin.vim

@@ -1,11 +1,11 @@
 " These commands create the option window.
 "
 " Maintainer:	The Vim Project <https://github.com/vim/vim>
-" Last Change:	2025 Jul 25
+" Last Change:	2025 Aug 07
 " Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 
 " If there already is an option window, jump to that one.
-let buf = bufnr('option-window')
+let buf = bufexists('option-window') ? bufnr('option-window') : -1
 if buf >= 0
   let winids = win_findbuf(buf)
   if len(winids) > 0