fix bug for static init

Ugh! Static initialization of instance variables is a very bad idea.

This fix is taken from the Chromium code-base. It includes their
double-fix for ARM.

* https://codereview.chromium.org/24984004
* https://src.chromium.org/viewvc/chrome?revision=226099&view=revision
* https://code.google.com/p/webrtc/issues/detail?id=1777
diff --git a/include/json/value.h b/include/json/value.h
index 92526a4..1ad006f 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -133,7 +133,7 @@
   typedef Json::LargestUInt LargestUInt;
   typedef Json::ArrayIndex ArrayIndex;
 
-  static const Value null;
+  static const Value& null;
   /// Minimum signed integer value that can be stored in a Json::Value.
   static const LargestInt minLargestInt;
   /// Maximum signed integer value that can be stored in a Json::Value.