Fix swap-pane crash

diff --git a/window.c b/window.c
index 1dd70c4..d54a6af 100644
--- a/window.c
+++ b/window.c
@@ -422,6 +422,9 @@ window_pane_active_lost(struct window_pane *wp, struct window_pane *nextwp)
 {
        struct layout_cell      *lc, *lc2;

+       if (nextwp->layout_cell->parent == NULL)
+               return;
+
        /* Save the target pane in its parent. */
        nextwp->layout_cell->parent->lastwp = nextwp;