patch 9.1.0672: marker folds may get corrupted on undo

Problem:  marker folds may get corrupted on undo (Yousef Mohammed)
Solution: when adjusting folds, make sure that line1 is the lower limit
          and line2 is the upper line limit. In particular, line2 should
          not be able to get smaller than line1.

fixes: #15455
closes: #15466

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_fold.vim b/src/testdir/test_fold.vim
index 7fb0043..17487a5 100644
--- a/src/testdir/test_fold.vim
+++ b/src/testdir/test_fold.vim
Binary files differ