ui: fix gtk clipboard clear assertion

ui: fix gtk clipboard clear assertion

When closing the QEMU Gtk display window, it can occasionaly warn:
qemu-system-x86_64: Gtk: gtk_clipboard_set_with_data: assertion 'targets != NULL' failed

#3  0x00007ffff4f02f22 in gtk_clipboard_set_with_data (clipboard=<optimized out>, targets=<optimized out>, n_targets=<optimized out>, get_func=<optimized out>, clear_func=<optimized out>, user_data=<optimized out>) at /usr/src/debug/gtk3-3.24.30-4.fc35.x86_64/gtk/gtkclipboard.c:672
#4  0x00007ffff552cd75 in gd_clipboard_update_info (gd=0x5555579a9e00, info=0x555557ba4b50) at ../ui/gtk-clipboard.c:98
#5  0x00007ffff552ce00 in gd_clipboard_notify (notifier=0x5555579aaba8, data=0x7fffffffd720) at ../ui/gtk-clipboard.c:128
#6  0x000055555603e0ff in notifier_list_notify (list=0x555556657470 <clipboard_notifiers>, data=0x7fffffffd720) at ../util/notify.c:39
#7  0x000055555594e8e0 in qemu_clipboard_update (info=0x555557ba4b50) at ../ui/clipboard.c:54
#8  0x000055555594e840 in qemu_clipboard_peer_release (peer=0x55555684a5b0, selection=QEMU_CLIPBOARD_SELECTION_PRIMARY) at ../ui/clipboard.c:40
#9  0x000055555594e786 in qemu_clipboard_peer_unregister (peer=0x55555684a5b0) at ../ui/clipboard.c:19
#10 0x000055555595f044 in vdagent_disconnect (vd=0x55555684a400) at ../ui/vdagent.c:852
#11 0x000055555595f262 in vdagent_chr_fini (obj=0x55555684a400) at ../ui/vdagent.c:908

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20211216083233.1166504-1-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>