Ensure Json::Value::null{,Ref} are initialized at compile time.

This bug was introduced in commit 48d9a92.

gcc now puts Json::Value::null{,Ref} into bss section:

  $ nm libjsoncpp.a | c++filt | grep Value::null
  0000000000000000 B Json::Value::null
  0000000000000008 B Json::Value::nullRef

When we access them in constructor of global objects, since the order of
initialization is unpredictable, we may dereference a null pointer.

After applying this patch, gcc puts them into rodata section:

  $ nm libjsoncpp.a | c++filt | grep Value::null
  0000000000000008 R Json::Value::null
  0000000000000010 R Json::Value::nullRef

Author: hchunhui

src/lib_json/json_value.cpp

@@ -34,8 +34,8 @@ const Value Value::null;
 #define ALIGNAS(byte_alignment)
 #endif
 static const unsigned char ALIGNAS(8) kNull[sizeof(Value)] = { 0 };
-const unsigned char& kNullRef = kNull[0];
-const Value& Value::nullRef = reinterpret_cast<const Value&>(kNullRef);
+const Value& Value::null = reinterpret_cast<const Value&>(kNull[0]);
+const Value& Value::nullRef = reinterpret_cast<const Value&>(kNull[0]);
 
 const Int Value::minInt = Int(~(UInt(-1) / 2));
 const Int Value::maxInt = Int(UInt(-1) / 2);