window: Fix logic for looking up widget default cursor

We may deref a NULL pointer if there is no grab and no focus widget.
diff --git a/clients/window.c b/clients/window.c
index d701889..e9b6a5e 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -2911,12 +2911,15 @@
 		widget = input->grab;
 	else
 		widget = input->focus_widget;
-	if (widget && widget->motion_handler)
-		cursor = widget->motion_handler(input->focus_widget,
-						input, time, sx, sy,
-						widget->user_data);
-	else
-		cursor = input->focus_widget->default_cursor;
+	if (widget) {
+		if (widget->motion_handler)
+			cursor = widget->motion_handler(input->focus_widget,
+							input, time, sx, sy,
+							widget->user_data);
+		else
+			cursor = widget->default_cursor;
+	} else
+		cursor = CURSOR_LEFT_PTR;
 
 	input_set_pointer_image(input, cursor);
 }