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

Author: cdunn2001

include/json/value.h

@@ -133,7 +133,7 @@ class JSON_API Value {
   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.

src/lib_json/json_value.cpp

@@ -25,7 +25,17 @@
 
 namespace Json {
 
-const Value Value::null;
+// This is a walkaround to avoid the static initialization of Value::null.
+// kNull must be word-aligned to avoid crashing on ARM.  We use an alignment of
+// 8 (instead of 4) as a bit of future-proofing.
+#if defined(__ARMEL__)
+#define ALIGNAS(byte_alignment) __attribute__((aligned(byte_alignment)))
+#else
+#define ALIGNAS(byte_alignment)
+#endif
+static const unsigned char ALIGNAS(8) kNull[sizeof(Value)] = {0};
+const Value& Value::null = reinterpret_cast<const Value&>(kNull);
+
 const Int Value::minInt = Int(~(UInt(-1) / 2));
 const Int Value::maxInt = Int(UInt(-1) / 2);
 const UInt Value::maxUInt = UInt(-1);