[Rd] Accessors returning (void *) 1 for zero-length vectors

[Iñaki Ucar]

Hi,

We were recently bitten by this [1] (see [2]), when an Rcpp user tried
to std::copy() a vector that may be of length zero and got an UB error
due to misaligned memory. Returning (void *) 1 seems like a very odd
choice to me. The comment suggests that this was a workaround for
Matrix. Wouldn't it be wiser to just fix Matrix and return NULL
instead, which is expected and easier to check and to handle?

[1] https://github.com/wch/r-source/commit/d00ce14c27db627d8644cce8551b792362cc46bf
[2] https://github.com/RcppCore/Rcpp/issues/1461

Best,
-- 
Iñaki Úcar