fix(runtime/tutor): don't try to close fold when there is none (#24953)

Problem: When double clicking a line starting with a #, the code assumes
there is a fold there and tries to close it, resulting in an error if
there isn't a fold.
Solution: Check foldlevel before performing "zc".

runtime/autoload/tutor.vim

@@ -29,7 +29,7 @@ function! tutor#MouseDoubleClick()
     if foldclosed(line('.')) > -1
         normal! zo
     else
-        if match(getline('.'), '^#\{1,} ') > -1
+        if match(getline('.'), '^#\{1,} ') > -1 && foldlevel(line('.')) > 0
             silent normal! zc
         else
             call tutor#FollowLink(0)