patch 8.2.0614: get ml_get error when deleting a line in 'completefunc'

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index e96d5fd..9890377 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -334,19 +334,17 @@
   endif
 endfunc
 
-" Test that nothing happens if the 'completefunc' opens
-" a new window (no completion, no crash)
+" Test that nothing happens if the 'completefunc' tries to open
+" a new window (fails to open window, continues)
 func Test_completefunc_opens_new_window_one()
   new
   let winid = win_getid()
   setlocal completefunc=DummyCompleteOne
   call setline(1, 'one')
   /^one
-  call assert_fails('call feedkeys("A\<C-X>\<C-U>\<C-N>\<Esc>", "x")', 'E839:')
-  call assert_notequal(winid, win_getid())
-  q!
+  call assert_fails('call feedkeys("A\<C-X>\<C-U>\<C-N>\<Esc>", "x")', 'E565:')
   call assert_equal(winid, win_getid())
-  call assert_equal('', getline(1))
+  call assert_equal('oneDEF', getline(1))
   q!
 endfunc