LibWeb/CSS: Interpret NaN as 0 when resolving alpha and rgb values

Fixes the crash in css/css-color/parsing/color-valid-hwb.html.

The crash was probably introduced in 248e4bb5, as it was the first
commit to VERIFY that the value given to `Color::with_opacity` were in
the correct range. As the values in color-valid-hwb.html were resolved
as NaN, the check never passed.

Libraries/LibWeb/CSS/StyleValues/CSSRGB.cpp

@@ -18,6 +18,8 @@ Color CSSRGB::to_color(Optional<Layout::NodeWithStyle const&>) const
     auto resolve_rgb_to_u8 = [](CSSStyleValue const& style_value) -> Optional<u8> {
         // <number> | <percentage> | none
         auto normalized = [](double number) {
+            if (isnan(number))
+                number = 0;
             return llround(clamp(number, 0.0, 255.0));
         };