Problem: a closing fold expression may unexpectedly start a new fold
when it should end a fold
(reported by Shota Nozaki)
Solution: if a fold hasn't started yet, do not immediately
start a new fold with level 1
(Shota Nozaki)
fixes: vim/vim#12768closes: vim/vim#137480689b87059
Co-authored-by: Shota Nozaki <emonkak@gmail.com>