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/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);