untag token upon removal from the precious list

Author: Enchufa2

src/barrier.cpp

@@ -2,7 +2,7 @@
 // barrier.cpp: Rcpp R/C++ interface class library -- write barrier
 //
 // Copyright (C) 2010 - 2020  Dirk Eddelbuettel and Romain Francois
-// Copyright (C) 2021         Dirk Eddelbuettel, Romain Francois and Iñaki Ucar
+// Copyright (C) 2021 - 2022  Dirk Eddelbuettel, Romain Francois and Iñaki Ucar
 //
 // This file is part of Rcpp.
 //
@@ -119,6 +119,7 @@ void Rcpp_precious_remove(SEXP token) {
     if (token == R_NilValue || TYPEOF(token) != LISTSXP) {
         return;
     }
+    SET_TAG(token, R_NilValue);
     SEXP before = CAR(token);
     SEXP after = CDR(token);
     SETCDR(before, after);