Bugfix: ipc window event crash with no window

Long description:

The log message at the top of ipc_send_window_event would segfault when
no xcb window is present, such as for a split container.

The log now shows the con id which is more relevant anyway.

To apply this patch, use:
curl http://cr.i3wm.org/patch/520/raw.patch | git am

@@ -1067,7 +1067,8 @@ void ipc_send_workspace_focus_event(Con *current, Con *old) {

  * also the window container, in "container".

  */

 void ipc_send_window_event(const char *property, Con *con) {

-    DLOG("Issue IPC window %s event for X11 window 0x%08x\n", property, con->window->id);

+    DLOG("Issue IPC window %s event (con = %p, window = 0x%08x)\n",

+            property, con, (con->window ? con->window->id : XCB_WINDOW_NONE));

     setlocale(LC_NUMERIC, "C");

     yajl_gen gen = ygenalloc();