6584008: jvmtiStringPrimitiveCallback should not be invoked when string value is null

Reviewed-by: sla, sspitsyn

src/share/vm/prims/jvmtiTagMap.cpp

@@ -1045,10 +1045,16 @@ static jint invoke_string_value_callback(jvmtiStringPrimitiveValueCallback cb,
 {
   assert(str->klass() == SystemDictionary::String_klass(), "not a string");
 
+  typeArrayOop s_value = java_lang_String::value(str);
+
+  // JDK-6584008: the value field may be null if a String instance is
+  // partially constructed.
+  if (s_value == NULL) {
+    return 0;
+  }
   // get the string value and length
   // (string value may be offset from the base)
   int s_len = java_lang_String::length(str);
-  typeArrayOop s_value = java_lang_String::value(str);
   int s_offset = java_lang_String::offset(str);
   jchar* value;
   if (s_len > 0) {