Replace NULL macros with nullptr literals.

Change-Id: Id2311cda59dd42c74b3ed54d3ff6cfd509012738
diff --git a/lib/ExecutionEngine/ELFObjectLoaderImpl.h b/lib/ExecutionEngine/ELFObjectLoaderImpl.h
index 8734558..1490bcb 100644
--- a/lib/ExecutionEngine/ELFObjectLoaderImpl.h
+++ b/lib/ExecutionEngine/ELFObjectLoaderImpl.h
@@ -40,7 +40,7 @@
 #endif
 
 public:
-  ELFObjectLoaderImpl() : ObjectLoaderImpl(), mObject(NULL), mSymTab(NULL) { }
+  ELFObjectLoaderImpl() : ObjectLoaderImpl(), mObject(nullptr), mSymTab(nullptr) { }
 
   virtual bool load(const void *pMem, size_t pMemSize);