COMP: Use nullptr instead of 0 or NULL

The check converts the usage of null pointer constants (eg. NULL, 0) to
use the new C++11 nullptr keyword.

SRCDIR=/Users/johnsonhj/src/jsoncpp #My local SRC
BLDDIR=/Users/johnsonhj/src/jsoncpp/cmake-build-debug/ #My local BLD

cd /Users/johnsonhj/src/jsoncpp/cmake-build-debug/
run-clang-tidy.py -extra-arg=-D__clang__ -checks=-*,modernize-use-nullptr  -header-filter=.* -fix
diff --git a/include/json/reader.h b/include/json/reader.h
index d8f1be1..b700d95 100644
--- a/include/json/reader.h
+++ b/include/json/reader.h
@@ -221,7 +221,7 @@
                                    Location end,
                                    unsigned int& unicode);
   bool
-  addError(const JSONCPP_STRING& message, Token& token, Location extra = 0);
+  addError(const JSONCPP_STRING& message, Token& token, Location extra = nullptr);
   bool recoverFromError(TokenType skipUntilToken);
   bool addErrorAndRecover(const JSONCPP_STRING& message,
                           Token& token,