patch 8.2.4428: crash when switching tabpage while in the cmdline window

Problem:    Crash when switching tabpage while in the cmdline window.
Solution:   Disallow switching tabpage when in the cmdline window.

src/window.c

@@ -110,6 +110,21 @@ log_frame_layout(frame_T *frame)
 }
 #endif
 
+/*
+ * Return the current window, unless in the cmdline window and "prevwin" is
+ * set, then return "prevwin".
+ */
+    win_T *
+prevwin_curwin(void)
+{
+    return
+#ifdef FEAT_CMDWIN
+	// In cmdwin, the alternative buffer should be used.
+	is_in_cmdwin() && prevwin != NULL ? prevwin :
+#endif
+	curwin;
+}
+
 /*
  * All CTRL-W window commands are handled here, called from normal_cmd().
  */
@@ -3927,6 +3942,14 @@ win_new_tabpage(int after)
     tabpage_T	*newtp;
     int		n;
 
+#ifdef FEAT_CMDWIN
+    if (cmdwin_type != 0)
+    {
+	emsg(_(e_invalid_in_cmdline_window));
+	return FAIL;
+    }
+#endif
+
     newtp = alloc_tabpage();
     if (newtp == NULL)
 	return FAIL;
@@ -4301,6 +4324,7 @@ goto_tabpage(int n)
 	text_locked_msg();
 	return;
     }
+    CHECK_CMDWIN;
 
     // If there is only one it can't work.
     if (first_tabpage->tp_next == NULL)
@@ -4368,6 +4392,8 @@ goto_tabpage_tp(
     int		trigger_enter_autocmds,
     int		trigger_leave_autocmds)
 {
+    CHECK_CMDWIN;
+
     // Don't repeat a message in another tab page.
     set_keep_msg(NULL, 0);