Eliminated null RCaller failure in tracing mode.

com.oracle.truffle.r.nodes.builtin/src/com/oracle/truffle/r/nodes/builtin/helpers/TraceHandling.java

 /*
- * Copyright (c) 2014, 2016, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2017, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -148,7 +148,7 @@ public class TraceHandling {
         protected String getCallSource(VirtualFrame frame) {
             RCaller caller = RArguments.getCall(frame);
             String callString;
-            if (caller != null) {
+            if (caller != null && caller.isValidCaller()) {
                 callString = getCallerSource(caller);
             } else {
                 callString = "<no source>";