css: Avoid a crash with relative colors

rgba(from @foo ...) would crash if @foo was not a named color.
Handle it as we do elsewhere, by returning NULL from resolve().

Test included.

Fixes: #6985

gtk/gtkcsscolorvalue.c

@@ -700,6 +700,8 @@ gtk_css_color_value_do_resolve (GtkCssValue          *color,
         GtkCssValue *vals[4];
 
         origin = gtk_css_color_value_do_resolve (color->relative.origin, context, current, cycle_list);
+        if (origin == NULL)
+          return NULL;
 
         for (guint i = 0; i < 4; i++)
           {

testsuite/css/style/bad-relative-color.css

@@ -0,0 +1,3 @@
+label {
+  color: oklab(from @foo max(l, 0.75) a b / alpha);
+}

testsuite/css/style/bad-relative-color.nodes

@@ -0,0 +1,4 @@
+window.background:dir(ltr)
+  box.horizontal:dir(ltr)
+    label:dir(ltr)
+      color: rgb(255,255,255); /* bad-relative-color.css:2:3-52 */

testsuite/css/style/bad-relative-color.ui

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <property name="decorated">0</property>
+    <child>
+      <object class="GtkBox">
+        <property name="visible">True</property>
+        <child>
+          <object class="GtkLabel">
+            <property name="visible">True</property>
+            <property name="label" translatable="yes">Hello World!</property>
+          </object>
+        </child>
+      </object>
+    </child>
+  </object>
+</interface>