Fix SIGSEGV on surface destroy

```
Program terminated with signal SIGSEGV, Segmentation fault.

warning: Section `.reg-xstate/3960717' in core file too small.
0  container_get_siblings (container=0x55bcde4797f0) at ../sway/tree/container.c:1228
1228		if (list_find(container->pending.workspace->tiling, container) != -1) {
[Current thread is 1 (Thread 0x7fa23b4a2940 (LWP 3960717))]
(gdb) bt full=
No symbol "full" in current context.
(gdb) bt full
0  container_get_siblings (container=0x55bcde4797f0) at ../sway/tree/container.c:1228
1  0x000055bcdb62c704 in edge_is_external (cont=0x55bcde4797f0, edge=(WLR_EDGE_TOP | WLR_EDGE_LEFT))
    at ../sway/input/seatop_default.c:54
        siblings = 0x55bcde4797f0
        index = 32766
        layout = L_NONE
        __PRETTY_FUNCTION__ = "edge_is_external"
2  0x000055bcdb62c96f in find_resize_edge (cont=0x55bcde4797f0, surface=0x0, cursor=0x55bcddd5c2e0)
    at ../sway/input/seatop_default.c:106
        edge = (WLR_EDGE_TOP | WLR_EDGE_LEFT)
3  0x000055bcdb620b3c in cursor_update_image (cursor=0x55bcddd5c2e0, node=0x55bcde4797f0) at ../sway/input/cursor.c:144
        edge = WLR_EDGE_NONE
4  0x000055bcdb62eb8f in handle_rebase (seat=0x55bcddd5a740, time_msec=488992944) at ../sway/input/seatop_default.c:773
        e = 0x55bcddd5c8e0
        cursor = 0x55bcddd5c2e0
        surface = 0x0
        sx = 0
        sy = 0
5  0x000055bcdb62c531 in seatop_rebase (seat=0x55bcddd5a740, time_msec=488992944) at ../sway/input/seat.c:1585
6  0x000055bcdb620a7d in cursor_rebase (cursor=0x55bcddd5c2e0) at ../sway/input/cursor.c:126
        time_msec = 488992944
7  0x000055bcdb620ac4 in cursor_rebase_all () at ../sway/input/cursor.c:136
        seat = 0x55bcddd5a740
8  0x000055bcdb61cc95 in transaction_apply (transaction=0x55bcde5b28c0) at ../sway/desktop/transaction.c:704
9  0x000055bcdb61ccdb in transaction_progress () at ../sway/desktop/transaction.c:716
10 0x000055bcdb61d1f9 in transaction_commit_pending () at ../sway/desktop/transaction.c:836
        transaction = 0x55bcde5b28c0
11 0x000055bcdb61d596 in _transaction_commit_dirty (server_request=true) at ../sway/desktop/transaction.c:912
12 0x000055bcdb61d5ac in transaction_commit_dirty () at ../sway/desktop/transaction.c:916
13 0x000055bcdb65f579 in view_unmap (view=0x55bcde2ff180) at ../sway/tree/view.c:847
        parent = 0x55bcde489010
        ws = 0x55bcdde19080
        seat = 0x55bcddd5a198
14 0x000055bcdb61e461 in handle_unmap (listener=0x55bcde2ff368, data=0x0) at ../sway/desktop/xdg_shell.c:394
        xdg_shell_view = 0x55bcde2ff180
        view = 0x55bcde2ff180
        __PRETTY_FUNCTION__ = "handle_unmap"
15 0x00007fa23c4ae87f in wlr_signal_emit_safe (signal=0x55bcde46cf38, data=0x0) at ../util/signal.c:29
        pos = 0x55bcde2ff368
        l = 0x55bcde2ff368
        cursor = {link = {prev = 0x55bcde2ff368, next = 0x7ffe240702a0}, notify = 0x7fa23c4ae7c9 <handle_noop>}
        end = {link = {prev = 0x7ffe24070280, next = 0x55bcde46cf38}, notify = 0x7fa23c4ae7c9 <handle_noop>}
16 0x00007fa23c47c3c7 in unmap_xdg_surface (surface=0x55bcde46ce30) at ../types/xdg_shell/wlr_xdg_surface.c:40
        __PRETTY_FUNCTION__ = "unmap_xdg_surface"
        popup = 0x55bcde46ce60
        popup_tmp = 0x55bcde46ce60
        configure = 0x7ffe24070360
        tmp = 0x55bcde488020
17 0x00007fa23c47cd47 in xdg_surface_role_precommit (wlr_surface=0x55bcde488020, state=0x55bcde4881a8)
    at ../types/xdg_shell/wlr_xdg_surface.c:330
        surface = 0x55bcde46ce30
18 0x00007fa23c4813b2 in surface_commit_state (surface=0x55bcde488020, next=0x55bcde4881a8) at ../types/wlr_compositor.c:407
        __PRETTY_FUNCTION__ = "surface_commit_state"
        invalid_buffer = false
        subsurface = 0xbd8e9aecae023300
--Type <RET> for more, q to quit, c to continue without paging--
19 0x00007fa23c48192a in surface_handle_commit (client=0x55bcde488850, resource=0x55bcde2fdb80) at ../types/wlr_compositor.c:523
        surface = 0x55bcde488020
20 0x00007fa23bb5ed4a in  () at /usr/lib/libffi.so.8
21 0x00007fa23bb5e267 in  () at /usr/lib/libffi.so.8
22 0x00007fa23c517323 in  () at /usr/lib/libwayland-server.so.0
23 0x00007fa23c5125cc in  () at /usr/lib/libwayland-server.so.0
24 0x00007fa23c5151ca in wl_event_loop_dispatch () at /usr/lib/libwayland-server.so.0
25 0x00007fa23c512d37 in wl_display_run () at /usr/lib/libwayland-server.so.0
26 0x000055bcdb616885 in server_run (server=0x55bcdb68c5c0 <server>) at ../sway/server.c:307
27 0x000055bcdb61594e in main (argc=3, argv=0x7ffe24070af8) at ../sway/main.c:433
```

It seems to be happening because of this set of events all happening
in the span of a single transaction:
1. You kill a tiled window that is the only window in a workplace.
2. Sway will destroy the workspace but not yet the container - this
   makes `con->pending.workspace` NULL.
3. Cursor glyphs get recomputed causing sway to recompute if the cursor
   is on a container edge. 
4. That computation causes an access to the NULL workspace. Crash.

sway/input/seatop_default.c

@@ -56,6 +56,9 @@ static bool edge_is_external(struct sway_container *cont, enum wlr_edges edge) {
 	while (cont) {
 		if (container_parent_layout(cont) == layout) {
 			list_t *siblings = container_get_siblings(cont);
+			if (!siblings) {
+				return false;
+			}
 			int index = list_find(siblings, cont);
 			if (index > 0 && (edge == WLR_EDGE_LEFT || edge == WLR_EDGE_TOP)) {
 				return false;