surface-actor-x11: Avoid crashing if leaked

There is no value in letting dispose crash when the x11_display pointer is NULL. A core dump doesn't help because the root cause is a leak elsewhere. So let's just give a warning about the leak instead of dumping core and generating crash reports.

Closes: #629 (closed) but most importantly prevents regressions of #629 (closed) if and when there are more leaks in future.

Note: @3v1n0 is going to propose a fix for the offending leak separately. That also hopefully can be seen as a fix for #629 (closed). But I suggest we need both.