patch 8.2.0142: possible to enter popup window with CTRL-W p

Problem:    Possible to enter popup window with CTRL-W p. (John Devin)
Solution:   Check entered window is not a popup window. (closes #5515)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 654501b..ebcc1f4 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2804,6 +2804,12 @@
   call term_sendkeys(buf, ":\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_previewpopup_8', {})
 
+  call term_sendkeys(buf, "\<C-W>p")
+  call VerifyScreenDump(buf, 'Test_popupwin_previewpopup_9', {})
+
+  call term_sendkeys(buf, ":call win_execute(popup_findpreview(), 'call popup_clear()')\<CR>")
+  call VerifyScreenDump(buf, 'Test_popupwin_previewpopup_10', {})
+
   call StopVimInTerminal(buf)
   call delete('Xtags')
   call delete('Xtagfile')