bug#56559: 29.0.50; Invalid CGContextSaveState on NS

[Gerd Möllmann]

In GNU Emacs 29.0.50 (build 1, aarch64-apple-darwin21.5.0, NS appkit-2113.50 
Version 12.4 (Build 21F79))
 of 2022-07-12 built on Mini.fritz.box
Repository revision: f209650e41d59356a90b5a602abc60c6783bc7b1
Repository branch: master
Windowing system distributor 'Apple', version 10.3.2113
System Description:  macOS 12.4

Configured using:
 'configure --with-native-compilation'

At various times, Emacs prints

2022-07-14 16:55:41.722952+0200 emacs[65244:294257] [emacs] CGContextSaveGState:
invalid context 0x0. If you want to see the backtrace, please set
CG_CONTEXT_SHOW_BACKTRACE environmental variable.

to the terminal from which it is started.  Setting the env variable,
gives the backtrace

2022-07-14 16:56:29.249179+0200 emacs[65299:295037] [emacs] 
CGContextSetLineJoin: invalid context 0x0. Backtrace:
  <+[NSBezierPath strokeRect:]+136>
   <ns_draw_window_cursor+5448>
    <display_and_set_cursor+4532>
     <update_window_cursor+1556>
      <update_cursor_in_window_tree+444>
       <gui_update_cursor+100>
        <ns_set_cursor_color+1140>
         <gui_set_frame_parameters+15768>
          <Fmodify_frame_parameters+856>
           <Finternal_set_lisp_face_attribute+54152>