patch 8.2.2898: QuitPre and ExitPre not triggered when GUI window is closed

Problem:    QuitPre and ExitPre not triggered when GUI window is closed.
Solution:   Call before_quit_autocmds(). (closes #8242)
diff --git a/src/gui.c b/src/gui.c
index c0374c5..2539a4f 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -857,9 +857,10 @@
     void
 gui_shell_closed(void)
 {
-    cmdmod_T	    save_cmdmod;
+    cmdmod_T	    save_cmdmod = cmdmod;
 
-    save_cmdmod = cmdmod;
+    if (before_quit_autocmds(curwin, TRUE, FALSE))
+	return;
 
     // Only exit when there are no changed files
     exiting = TRUE;