css: Fix a crash in variables

We were not careful enough when checking for reference cycles.

Fixes: #6935

Test included.

gtk/gtkcssvariableset.c

@@ -139,6 +139,10 @@ check_references (GtkCssVariableSet   *self,
 
           ref_value = g_hash_table_lookup (self->variables, GINT_TO_POINTER (ref_id));
 
+          /* This variable was already removed, no point in checking further */
+          if (!ref_value)
+            return FALSE;
+
           if (check_variable (self, unvisited_variables, stack, ref_id, ref_value))
             return TRUE;
         }

testsuite/css/style/variables2.css

@@ -0,0 +1,11 @@
+:root {
+  --b: var(--a);
+}
+
+:root {
+  --a: var(--a, red);
+}
+
+* {
+ background: var(--b);
+}

testsuite/css/style/variables2.nodes

@@ -0,0 +1,9 @@
+window.background:dir(ltr)
+  background-color: rgba(0,0,0,0); /* variables2.css:10:2-23 */
+  background-clip: border-box; /* variables2.css:10:2-23 */
+  background-origin: padding-box; /* variables2.css:10:2-23 */
+  background-size: auto; /* variables2.css:10:2-23 */
+  background-position: left top; /* variables2.css:10:2-23 */
+  background-repeat: repeat; /* variables2.css:10:2-23 */
+  background-image: none; /* variables2.css:10:2-23 */
+  --b: var(--a); /* variables2.css:2:8-16 */

testsuite/css/style/variables2.ui

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <property name="decorated">0</property>
+  </object>
+</interface>