fix overflow in MatrixColumn ctor

[thirdwing]

This is for the error reported in mailing list: https://www.mail-archive.com/rcpp-devel@lists.r-forge.r-project.org/msg08582.html

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::IntegerMatrix makeMat(int nrow=18, int ncol=136633572){
  Rcpp::IntegerMatrix mat(nrow, ncol);
  for (int i = 0; i < ncol; ++i) {
    Rcpp::MatrixColumn<INTSXP> col = mat.column(i);
    for (int j = 0; j < nrow; ++j){
      col[j] = 1;
    }
  }
  return mat;
}
/*
> Rcpp::sourceCpp("test.cpp")
> mat <- makeMat()

 *** caught segfault ***
address 0x7f9d542ee088, cause 'memory not mapped'

Traceback:
 1: .Primitive(".Call")(<pointer: 0x7fa1b368b2a0>, nrow, ncol)
 2: makeMat()
*/

In MatrixColumn's ctor, even i and n are both int, i * n can still cause an overflow, which will lead to a segfault.

[eddelbuettel]

👍 from a casual glance -- and thanks for getting to it so quickly!