Fix rchk and asan/ubsan errors

[paleolimbot]

Identified by @edzer here: #186 (comment)

[paleolimbot]

The rchk issue appears to be that it thinks the deleter for (a unique pointer to a) RGeography is an allocating function?

Function cpp_s2_cell_center(Rcpp::Vector<14, Rcpp::PreserveStorage>)::Op::processCell(S2CellId, long)
  [UP] unprotected variable <unnamed var:   %4 = alloca %struct.SEXPREC*, align 8> while calling allocating function std::__1::unique_ptr<RGeography, std::__1::default_delete<RGeography> >::~unique_ptr() /home/docker/R-svn/packages/build/Xnv2po3A/s2/src/s2-cell.cpp:357

[paleolimbot]

I had to ask in r-lib for help...the pattern that triggers this is if (condition) { return some_sexp_from_a_unique_ptr; } else { return R_NilValue; } and is likely a false error (but we still have to find a workaround...probably just go back to using Rcpp::XPtr<>()).