Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/rsContext.h b/rsContext.h
index f750670..2e59cc7 100644
--- a/rsContext.h
+++ b/rsContext.h
@@ -234,7 +234,7 @@
     static void printWatchdogInfo(void *ctx);
 
     void dumpDebug() const;
-    void setError(RsError e, const char *msg = NULL) const;
+    void setError(RsError e, const char *msg = nullptr) const;
 
     mutable const ObjectBase * mObjHead;