patch 9.0.0822: crash when dragging the statusline with a mapping

Problem:    Crash when dragging the statusline with a mapping.
Solution:   Check for valid window pointer. (issue #11427)

src/testdir/test_mapping.vim

@@ -1648,6 +1648,24 @@ func Test_mouse_drag_mapped_start_select()
   set mouse&
 endfunc
 
+func Test_mouse_drag_statusline()
+  set laststatus=2
+  set mouse=a
+  func ClickExpr()
+      call test_setmouse(&lines - 1, 1)
+        return "\<LeftMouse>"
+  endfunc
+  func DragExpr()
+      call test_setmouse(&lines - 2, 1)
+        return "\<LeftDrag>"
+  endfunc
+  nnoremap <expr> <F2> ClickExpr()
+  nnoremap <expr> <F3> DragExpr()
+
+  " this was causing a crash in win_drag_status_line()
+  call feedkeys("\<F2>:tabnew\<CR>\<F3>", 'tx')
+endfunc
+
 " Test for mapping <LeftDrag> in Insert mode
 func Test_mouse_drag_insert_map()
   set mouse=a