ui/gtk: Fix gd_refresh_rate_millihz() when widget window is not realized

ui/gtk: Fix gd_refresh_rate_millihz() when widget window is not realized

gtk_widget_get_window() returns NULL if the widget's window is not
realized, and QEMU crashes. Example under gtk 3.22.30 (mate 1.20.1):

  qemu-system-x86_64: Gdk: gdk_window_get_origin: assertion 'GDK_IS_WINDOW (window)' failed
  (gdb) bt
  #0  0x00007ffff496cf70 in gdk_window_get_origin () from /usr/lib64/libgdk-3.so.0
  #1  0x00007ffff49582a0 in gdk_display_get_monitor_at_window () from /usr/lib64/libgdk-3.so.0
  #2  0x0000555555bb73e2 in gd_refresh_rate_millihz (window=0x5555579d6280) at ui/gtk.c:1973
  #3  gd_vc_gfx_init (view_menu=0x5555579f0590, group=0x0, idx=0, con=<optimized out>, vc=0x5555579d4a90, s=0x5555579d49f0) at ui/gtk.c:2048
  #4  gd_create_menu_view (s=0x5555579d49f0) at ui/gtk.c:2149
  #5  gd_create_menus (s=0x5555579d49f0) at ui/gtk.c:2188
  #6  gtk_display_init (ds=<optimized out>, opts=0x55555661ed80 <dpy>) at ui/gtk.c:2256
  #7  0x000055555583d5a0 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4358

Fixes: c4c00922cc and 28b58f19d2 (display/gtk: get proper refreshrate)
Reported-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Jan Kiszka <jan.kiszka@web.de>
Message-id: 20200208161048.11311-3-f4bug@amsat.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>