WaE: implicit conversion of NULL constant to 'boost::detail::sp_nullptr_t'

Change-Id: I71923bdeb7b66662f114b3e72d9e3846f8b9ff92

vcl/opengl/gdiimpl.cxx

@@ -134,7 +134,7 @@ bool OpenGLSalGraphicsImpl::AcquireContext( bool bOffscreen )
         pContext =CreateWinContext();
 
     mpContext.reset(pContext);
-    return (mpContext != NULL);
+    return (mpContext != nullptr);
 }
 
 bool OpenGLSalGraphicsImpl::ReleaseContext()