libview: Add check for pageless documents to ev_view_accessible_focus_changed

Without this check, Evince will crash when opening a document that, for
whatever reason, has no pages.

https://bugzilla.gnome.org/show_bug.cgi?id=731046

origin commit:
https://git.gnome.org/browse/evince/commit/?id=c0ecc95

Author: joanmarie

libview/ev-view-accessible.c

@@ -448,7 +448,7 @@ ev_view_accessible_focus_changed (GtkWidget        *widget,
 	g_return_val_if_fail (EV_IS_VIEW (widget), FALSE);
 	g_return_val_if_fail (EV_IS_VIEW_ACCESSIBLE (self), FALSE);
 
-	if (self->priv->children == NULL)
+	if (self->priv->children == NULL || self->priv->children->len == 0)
 		return FALSE;
 
 	page_accessible = g_ptr_array_index (self->priv->children,