wlr_primary_selection: fix use-after-free when cancelling source

seat->primary_election_source_destroy points to the source that just got
freed by the cancel.

==7843==ERROR: AddressSanitizer: heap-use-after-free on address 0x60b0004269b0 at pc 0x7fb95bf4ccd0 bp 0x7ffd75013940 s
p 0x7ffd75013930
WRITE of size 8 at 0x60b0004269b0 thread T0
    #0 0x7fb95bf4cccf in wl_list_remove ../util/signal.c:55
    #1 0x7fb95bf3f4c6 in wlr_seat_set_primary_selection ../types/wlr_primary_selection.c:238
    #2 0x7fb95becb1a7 in xwm_handle_selection_event ../xwayland/selection/selection.c:124
    #3 0x7fb95bed2e5d in x11_event_handler ../xwayland/xwm.c:1139
    #4 0x7fb95c1bdf01 in wl_event_loop_dispatch src/event-loop.c:641
    #5 0x7fb95c1bc601 in wl_display_run src/wayland-server.c:1260
    #6 0x40a2f4 in main ../sway/main.c:433
    #7 0x7fb95b69718a in __libc_start_main (/lib64/libc.so.6+0x2318a)
    #8 0x40b749 in _start (/opt/wayland/bin/sway+0x40b749)

0x60b0004269b0 is located 64 bytes inside of 112-byte region [0x60b000426970,0x60b0004269e0)
freed by thread T0 here:
    #0 0x7fb95e0ad880 in __interceptor_free (/lib64/libasan.so.5+0xee880)
    #1 0x7fb95bf3f49e in wlr_seat_set_primary_selection ../types/wlr_primary_selection.c:236
    #2 0x7fb95becb1a7 in xwm_handle_selection_event ../xwayland/selection/selection.c:124
    #3 0x7fb95bed2e5d in x11_event_handler ../xwayland/xwm.c:1139
    #4 0x7fb95c1bdf01 in wl_event_loop_dispatch src/event-loop.c:641

previously allocated by thread T0 here:
    #0 0x7fb95e0ade50 in calloc (/lib64/libasan.so.5+0xeee50)
    #1 0x7fb95bec7ad6 in xwm_selection_get_targets ../xwayland/selection/incoming.c:355
    #2 0x7fb95bec7ad6 in xwm_handle_selection_notify ../xwayland/selection/incoming.c:402
    #3 0x7fb95becb1a7 in xwm_handle_selection_event ../xwayland/selection/selection.c:124
    #4 0x7fb95bed2e5d in x11_event_handler ../xwayland/xwm.c:1139
    #5 0x7fb95c1bdf01 in wl_event_loop_dispatch src/event-loop.c:641

SUMMARY: AddressSanitizer: heap-use-after-free ../util/signal.c:55 in wl_list_remove
Shadow bytes around the buggy address:
  0x0c168007cce0: fd fd fd fa fa fa fa fa fa fa fa fa fd fd fd fd
  0x0c168007ccf0: fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa fa
  0x0c168007cd00: fa fa fd fd fd fd fd fd fd fd fd fd fd fd fd fa
  0x0c168007cd10: fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd
  0x0c168007cd20: fd fd fd fd fd fa fa fa fa fa fa fa fa fa fd fd
=>0x0c168007cd30: fd fd fd fd fd fd[fd]fd fd fd fd fd fa fa fa fa
  0x0c168007cd40: fa fa fa fa fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c168007cd50: fd fa fa fa fa fa fa fa fa fa fd fd fd fd fd fd
  0x0c168007cd60: fd fd fd fd fd fd fd fa fa fa fa fa fa fa fa fa
  0x0c168007cd70: fd fd fd fd fd fd fd fd fd fd fd fd fd fa fa fa
  0x0c168007cd80: fa fa fa fa fa fa fd fd fd fd fd fd fd fd fd fd

types/wlr_primary_selection.c

@@ -229,9 +229,9 @@ void wlr_seat_set_primary_selection(struct wlr_seat *seat,
 	}
 
 	if (seat->primary_selection_source) {
+		wl_list_remove(&seat->primary_selection_source_destroy.link);
 		seat->primary_selection_source->cancel(seat->primary_selection_source);
 		seat->primary_selection_source = NULL;
-		wl_list_remove(&seat->primary_selection_source_destroy.link);
 	}
 
 	seat->primary_selection_source = source;