Fix crash when closing last child of a tabbed container

The crash only occurs if the mouse cursor is above the tabbed container
when the last child is closed.

Introduced in 03d49490cc, over a week ago
and unnoticed until now :O

The above commit changes the behaviour of a focus change. When you
change focus, it sends pointer motion which makes the client set a new
cursor image. We already had this behaviour for workspace switching, but
this commit adds it for view switching too, such as in a tabbed
container or when closing a view.

The sequence of events that leads to the crash is:

* The last child of a tabbed container unmaps, which triggers a
`destroy` event before we've cleaned up the child or reaped the tabbed
container.
* The seat code listens to the `destroy` event and removes the seat
container from the focus stack. As part of this, it decides to set focus
to the parent (my fix alters this decision).
* When setting focus to the new parent, the container motion is sent as
per the previously mentioned commit.
* The motion code uses `container_at`, which encounters the tabbed
container and its child in a half destroyed state, and everything blows
up from there.

`con->parent` is needed because scratchpad containers don't have parents
if they're hidden, so this probably fixes a crash when a hidden
scratchpad container closes too.

The `con->parent->children->length > 1` check should catch any cases
where the parent is about to be reaped.

sway/input/seat.c

@@ -181,6 +181,7 @@ static void handle_seat_container_destroy(struct wl_listener *listener,
 	bool set_focus =
 		focus != NULL &&
 		(focus == con || container_has_child(con, focus)) &&
+		con->parent && con->parent->children->length > 1 &&
 		con->type != C_WORKSPACE;
 
 	seat_container_destroy(seat_con);