wm: fixed segmentation fault in wm_find_leader

[Monsterovich]

I triggered this by simply opening OpenMPT, which first shows the application logo and then opens the main window.

Program received signal SIGSEGV, Segmentation fault.
0x00005555555db17d in wm_find_leader (wm=0x5555556c1470, node=0x0) at ../src/wm/wm.c:212
212             if (node->leader_final != NULL) {
(gdb) back
#0  0x00005555555db17d in wm_find_leader (wm=0x5555556c1470, node=0x0) at ../src/wm/wm.c:212
#1  0x00005555555db28b in wm_find_leader (wm=0x5555556c1470, node=0x5555556ebab0) at ../src/wm/wm.c:229
#2  0x00005555555db39e in wm_refresh_leaders (wm=0x5555556c1470) at ../src/wm/wm.c:251
#3  0x0000555555593b97 in refresh_windows (ps=0x5555556b6580) at ../src/picom.c:1625
#4  0x0000555555593d3b in handle_pending_updates (ps=0x5555556b6580, delta_t=0.014999999999999999) at ../src/picom.c:1662
#5  0x00005555555942b0 in draw_callback_impl (loop=0x7ffff7ea8720, ps=0x5555556b6580, revents=256) at ../src/picom.c:1732
#6  0x0000555555594e13 in draw_callback (loop=0x7ffff7ea8720, w=0x5555556b65e0, revents=256) at ../src/picom.c:1933
#7  0x00007ffff7e9c773 in ev_invoke_pending () from /lib/x86_64-linux-gnu/libev.so.4
#8  0x00007ffff7ea0041 in ev_run () from /lib/x86_64-linux-gnu/libev.so.4
#9  0x00005555555995eb in session_run (ps=0x5555556b6580) at ../src/picom.c:2746
#10 0x0000555555599972 in main (argc=1, argv=0x7fffffffe358) at ../src/picom.c:2861

[yshui]

thanks for reporting. so basically what happened is we tried to refresh leader before the window tree is consistent.

your fix is not sufficient, and there are other places where crash could happen too, e.g. ... (sorry, wrong example).

but the more important problem here is the window's leader will be wrong.

i will try to find the correct fix. closing this for now.