fix unbound effectively final cells

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/cell/ReadLocalCellNode.java

@@ -81,7 +81,11 @@ public ReadFromCellNode(boolean isFreeVar, Object identifier) {
         Object readFinal(@SuppressWarnings("unused") PCell cell,
                         @SuppressWarnings("unused") @Cached("cell") PCell cachedCell,
                         @Cached("cell.getRef()") Object ref) {
-            return ref;
+            if (ref != null) {
+                return ref;
+            } else {
+                throw raiseUnbound();
+            }
         }
 
         @Specialization(replaces = "readFinal")