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_ex_mode.vim b/src/testdir/test_ex_mode.vim
index 9f66e3f..d23e0ad 100644
--- a/src/testdir/test_ex_mode.vim
+++ b/src/testdir/test_ex_mode.vim
@@ -158,13 +158,13 @@
 func Test_ex_mode_errors()
   " Not allowed to enter ex mode when text is locked
   au InsertCharPre <buffer> normal! gQ<CR>
-  let caught_e523 = 0
+  let caught_e565 = 0
   try
     call feedkeys("ix\<esc>", 'xt')
-  catch /^Vim\%((\a\+)\)\=:E523/ " catch E523
-    let caught_e523 = 1
+  catch /^Vim\%((\a\+)\)\=:E565/ " catch E565
+    let caught_e565 = 1
   endtry
-  call assert_equal(1, caught_e523)
+  call assert_equal(1, caught_e565)
   au! InsertCharPre
 endfunc