fix(exit): skip unnecessary steps in TUI preserve_exit() (#21897)

This prevents the TUI from doing unexpected things when receiving a
deadly signal or running out of memory.

src/nvim/main.c

@@ -811,6 +811,11 @@ void preserve_exit(void)
   really_exiting = true;
   // Ignore SIGHUP while we are already exiting. #9274
   signal_reject_deadly();
+
+  if (ui_client_channel_id) {
+    os_exit(1);
+  }
+
   os_errmsg(IObuff);
   os_errmsg("\n");
   ui_flush();

src/nvim/tui/tui.c

@@ -458,6 +458,9 @@ static void tui_terminal_stop(TUIData *tui)
 
 void tui_stop(TUIData *tui)
 {
+  if (tui->stopped) {
+    return;
+  }
   tui_terminal_stop(tui);
   stream_set_blocking(tui->input.in_fd, true);   // normalize stream (#2598)
   tinput_destroy(&tui->input);