Fix crash in parse_border()

Fix crash in parse_border()

The bug was introduced in commit:
	9b7640b8 by Benjamin Otte, 2012-03-26 17:24:02
	styleproperty: Make _gtk_style_property_parse_value() return a CssValue

In that commit, `values` changed from `GValue*` to `GtkCssValue**`,
but one `!G_IS_VALUE (&values[8])` was left untouched. As a result,
if `border` shorthand contains anything after color, it might crash,
depending on memory layout.

New test included.

Fixes: #751