patch 8.2.1972: crash when recreating nested fold

Problem:    Crash when recreating nested fold.
Solution:   Check for empty growarray. (closes #7278)
diff --git a/src/testdir/test_fold.vim b/src/testdir/test_fold.vim
index f0bd6e4..1377983 100644
--- a/src/testdir/test_fold.vim
+++ b/src/testdir/test_fold.vim
@@ -844,4 +844,14 @@
   bwipe!
 endfunc
 
+" this was crashing
+func Test_fold_create_delete_create()
+  new
+  fold
+  fold
+  normal zd
+  fold
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab