patch 8.0.1432: after ":copen" can't get the window-ID of the quickfix window

Problem:    After ":copen" can't get the window-ID of the quickfix window.
            (FalacerSelene)
Solution:   Make it work without a quickfix list.  Add a test. (Yegappan
            Lakshmanan, closes #2541)
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index fb78f69..71aa7d5 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -2906,6 +2906,12 @@
   call assert_equal(0, g:Xgetlist({'changedtick' : 0}).changedtick)
   call assert_equal({'context' : '', 'id' : 0, 'idx' : 0, 'items' : [], 'nr' : 0, 'size' : 0, 'title' : '', 'winid' : 0, 'changedtick': 0}, g:Xgetlist({'all' : 0}))
 
+  " Quickfix window with empty stack
+  silent! Xopen
+  let qfwinid = (a:cchar == 'c') ? win_getid() : 0
+  call assert_equal(qfwinid, g:Xgetlist({'winid' : 0}).winid)
+  Xclose
+
   " Empty quickfix list
   Xexpr ""
   call assert_equal('', g:Xgetlist({'context' : 0}).context)