misc fixes

[gtk-perl schmorp de]

> There is some other similar small breakage, but I forgot to fix that when

Haven't stumbled over htem yet, but thinking about it, it seems
SvGdkGCValues and similar structure-setting fucntions are all broken in
the same way: there is no way to reset pointers, as Gtk2 enforces them to
be non-NULL.

Is this correct?

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