patch 8.1.2037: can call win_gotoid() in cmdline window

Problem:    Can call win_gotoid() in cmdline window.
Solution:   Disallow switching windows. (Yasuhiro Matsumoto, closes #4940)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index f8d6125..98768b5 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -734,3 +734,13 @@
 
   let &encoding = encoding_save
 endfunc
+
+func Test_cmdwin_bug()
+  let winid = win_getid()
+  sp
+  try
+    call feedkeys("q::call win_gotoid(" .. winid .. ")\<CR>:q\<CR>", 'x!')
+  catch /^Vim\%((\a\+)\)\=:E11/
+  endtry
+  bw!
+endfunc