safer DataFrame(SEXP) ctor. reported in tidyverse/dplyr#998

Author: romainfrancois

inst/include/Rcpp/DataFrame.h

@@ -2,7 +2,7 @@
 //
 // DataFrame.h: Rcpp R/C++ interface class library -- data frames
 //
-// Copyright (C) 2010 - 2014  Dirk Eddelbuettel and Romain Francois
+// Copyright (C) 2010 - 2015  Dirk Eddelbuettel and Romain Francois
 //
 // This file is part of Rcpp.
 //
@@ -40,7 +40,7 @@ namespace Rcpp{
         typedef Vector<VECSXP, StoragePolicy> Parent ;
 
         DataFrame_Impl() : Parent( internal::empty_data_frame() ){}
-        DataFrame_Impl(SEXP x) {
+        DataFrame_Impl(SEXP x) : Parent(x) {
             set__(x);
         }
         DataFrame_Impl( const DataFrame_Impl& other){