jump-or-exec fails when a window without WM_CLASS property exist

[amateur <tianlei zhao gmail com>]

Hi all,

I have recently trasnfered from fvwm to sawfish. I really like it for
its programmability.

However, I encounter a problem. When I open a window without WM_CLASS
property, like xev, the jump-or-exec functionality fails. The problem
lies in the usage of
    (string-match "xxx" (window-class w))
where (window-class w) returns nil.

It's strange that I thought a nil value should just silently returns a
unmatch. However, sawfish generates an error and stop working. Can
anyone look at this and fix it?

Thank you.

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