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

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>