Allocation failure fix and test

expression_reference.cpp

@@ -0,0 +1,8 @@
+#include "expression_reference.h"
+#include "allocation_failed_expression_node.h"
+
+template<>
+TreeNode * ExpressionRef::failedAllocationNode() {
+  static AllocationFailedExpressionRef FailureRef;
+  return FailureRef.node();
+}