widget: Protect fudge_pango_colors() against all-inclusive PangoAttributes

PangoAttribute documentation says "By default an attribute will have an
all-inclusive range of [0,G_MAXUINT]". It seems legal to get that from IMs
(referring to the pre-edit string), however the only caller of this
function just relies on the attribute being within the VteCells range,
leading to crashes.

https://bugzilla.gnome.org/show_bug.cgi?id=793480