patch 9.0.0820: memory leak with empty shell command

Problem:    Memory leak with empty shell command.
Solution:   Free the empty string.
diff --git a/src/errors.h b/src/errors.h
index 7aeca75..d3bca1a 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3330,4 +3330,6 @@
 	INIT(= N_("E1306: Loop nesting too deep"));
 EXTERN char e_argument_nr_trying_to_modify_const_str[]
 	INIT(= N_("E1307: Argument %d: Trying to modify a const %s"));
+EXTERN char e_cannot_resize_window_in_another_tab_page[]
+	INIT(= N_("E1308: Cannot resize a window in another tab page"));
 #endif
diff --git a/src/evalwindow.c b/src/evalwindow.c
index 3d3301f..98c7ff3 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -914,6 +914,11 @@
     wp = find_win_by_nr_or_id(&argvars[0]);
     if (wp == NULL || win_valid_popup(wp))
 	return;
+    if (!win_valid(wp))
+    {
+	emsg(_(e_cannot_resize_window_in_another_tab_page));
+	return;
+    }
 
     offset = (int)tv_get_number(&argvars[1]);
     win_drag_status_line(wp, offset);
diff --git a/src/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 6368889..8a71913 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -1545,23 +1545,33 @@
     call assert_true(id->win_move_statusline(-offset))
     call assert_equal(h, winheight(id))
   endfor
+
   " check that win_move_statusline doesn't error with offsets beyond moving
   " possibility
   call assert_true(win_move_statusline(id, 5000))
   call assert_true(winheight(id) > h)
   call assert_true(win_move_statusline(id, -5000))
   call assert_true(winheight(id) < h)
+
   " check that win_move_statusline returns false for an invalid window
   wincmd =
   let h = winheight(0)
   call assert_false(win_move_statusline(-1, 1))
   call assert_equal(h, winheight(0))
+
   " check that win_move_statusline returns false for a popup window
   let id = popup_create(['hello', 'world'], {})
   let h = winheight(id)
   call assert_false(win_move_statusline(id, 1))
   call assert_equal(h, winheight(id))
   call popup_close(id)
+
+  " check that using another tabpage fails without crash
+  let id = win_getid()
+  tabnew
+  call assert_fails('call win_move_statusline(id, -1)', 'E1308:')
+  tabclose
+
   %bwipe!
 endfunc
 
diff --git a/src/version.c b/src/version.c
index 2d2ae6a..58f17e1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    821,
+/**/
     820,
 /**/
     819,