core: fixed segmentation fault in handle_pending_updates

[Monsterovich]

You can invoke it if you quickly switch between the full-screen window and other windows. Most likely the callback call occurs at the moment when the backend has already been destroyed.

Program received signal SIGSEGV, Segmentation fault.
handle_pending_updates (ps=0x5555556b4530, delta_t=0) at ../src/picom.c:1668
1668					ps->backend_data->ops.release_image(ps->backend_data,
(gdb) back
#0  handle_pending_updates (ps=0x5555556b4530, delta_t=0) at ../src/picom.c:1668
#1  0x0000555555594061 in draw_callback_impl (loop=0x7ffff7ea8720, ps=0x5555556b4530, revents=256) at ../src/picom.c:1724
#2  0x0000555555594bc4 in draw_callback (loop=0x7ffff7ea8720, w=0x5555556b4590, revents=256) at ../src/picom.c:1925
#3  0x00007ffff7e9c773 in ev_invoke_pending () from /lib/x86_64-linux-gnu/libev.so.4
#4  0x00007ffff7ea0041 in ev_run () from /lib/x86_64-linux-gnu/libev.so.4
#5  0x00005555555993cb in session_run (ps=0x5555556b4530) at ../src/picom.c:2738
#6  0x0000555555599752 in main (argc=1, argv=0x7fffffffde28) at ../src/picom.c:2853
(gdb) 0
Undefined command: "0".  Try "help".
(gdb) step 0

Program received signal SIGSEGV, Segmentation fault.
handle_pending_updates (ps=0x5555556b4530, delta_t=0) at ../src/picom.c:1668
1668					ps->backend_data->ops.release_image(ps->backend_data,
(gdb) list
1663				free(w->running_animation_instance);
1664				w->running_animation_instance = NULL;
1665				w->in_openclose = false;
1666				if (w->saved_win_image != NULL) {
1667					printf("%c\n", ps->backend_data);
1668					ps->backend_data->ops.release_image(ps->backend_data,
1669					                                    w->saved_win_image);
1670					w->saved_win_image = NULL;
1671				}
1672				if (w->state == WSTATE_UNMAPPED) {
(gdb) w
watch           wh              whatis          where           while           while-stepping  winheight       with            ws              
(gdb) p /s w
$1 = (struct win *) 0x55555570cff0
(gdb) p /s ps
$2 = (struct session *) 0x5555556b4530
(gdb) p /s ps->backend_data
$3 = (backend_t *) 0x0

[yshui]

Thank you!