vim-patch:8.1.1968: crash when using nested map() (#27029)

Problem:    Crash when using nested map().
Solution:   Clear the pointer in prepare_vimvar(). (Ozaki Kiichi,
            closes vim/vim#4890, closes vim/vim#4891)

https://github.com/vim/vim/commit/27da7de7c547dbf983ed7dd901ea59be4e7c9ab2

Cherry-pick Test_filter_map_nested() from patch 8.1.1964.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

2 files changed, 13 insertions, 0 deletions

diff --git a/test/old/testdir/test_filter_map.vim b/test/old/testdir/test_filter_map.vim
index 7658797759..fb435f4291 100644
--- a/test/old/testdir/test_filter_map.vim
+++ b/test/old/testdir/test_filter_map.vim
@@ -56,6 +56,15 @@ func Test_filter_map_list_expr_funcref()
   call assert_equal([0, 2, 4, 6], map([1, 2, 3, 4], function('s:filter4')))
 endfunc
 
+func Test_filter_map_nested()
+  let x = {"x":10}
+  let r = map(range(2), 'filter(copy(x), "1")')
+  call assert_equal([x, x], r)
+
+  let r = map(copy(x), 'filter(copy(x), "1")')
+  call assert_equal({"x": x}, r)
+endfunc
+
 " dict with funcref
 func Test_filter_map_dict_expr_funcref()
   let dict = {"foo": 1, "bar": 2, "baz": 3}
diff --git a/test/old/testdir/test_functions.vim b/test/old/testdir/test_functions.vim
index 938928cd0b..3f9ac94782 100644
--- a/test/old/testdir/test_functions.vim
+++ b/test/old/testdir/test_functions.vim
@@ -2671,6 +2671,10 @@ func Test_readdir()
   let files = readdir('Xdir', {x -> len(add(l, x)) == 2 ? -1 : 1})
   call assert_equal(1, len(files))
 
+  " Nested readdir() must not crash
+  let files = readdir('Xdir', 'readdir("Xdir", "1") != []')
+  call sort(files)->assert_equal(['bar.txt', 'dir', 'foo.txt'])
+
   eval 'Xdir'->delete('rf')
 endfunc