patch 9.0.0824: crash when using win_move_separator() in other tab page

Problem:    Crash when using win_move_separator() in other tab page.
Solution:   Check for valid window in current tab page.
            (closes #11479, closes #11427)
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 8a71913..0d4c278 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -1483,23 +1483,33 @@
   call assert_equal(w0, winwidth(0))
   call assert_true(win_move_separator(0, -1))
   call assert_equal(w0, winwidth(0))
+
   " check that win_move_separator doesn't error with offsets beyond moving
   " possibility
   call assert_true(win_move_separator(id, 5000))
   call assert_true(winwidth(id) > w)
   call assert_true(win_move_separator(id, -5000))
   call assert_true(winwidth(id) < w)
+
   " check that win_move_separator returns false for an invalid window
   wincmd =
   let w = winwidth(0)
   call assert_false(win_move_separator(-1, 1))
   call assert_equal(w, winwidth(0))
+
   " check that win_move_separator returns false for a popup window
   let id = popup_create(['hello', 'world'], {})
   let w = winwidth(id)
   call assert_false(win_move_separator(id, 1))
   call assert_equal(w, winwidth(id))
   call popup_close(id)
+
+  " check that using another tabpage fails without crash
+  let id = win_getid()
+  tabnew
+  call assert_fails('call win_move_separator(id, -1)', 'E1308:')
+  tabclose
+
   %bwipe!
 endfunc