fix missing R_UnboundValue in old path

Author: Enchufa2

inst/include/Rcpp/Function.h

@@ -72,12 +72,13 @@ namespace Rcpp{
         Function_Impl(const std::string& name, const std::string& ns) {
 #if R_VERSION < R_Version(4,5,0)
             Shield<SEXP> env(Rf_findVarInFrame(R_NamespaceRegistry, Rf_install(ns.c_str())));
+            if (env == R_UnboundValue)
+                stop("there is no namespace called \"%s\"", ns);
 #else
             Shield<SEXP> env(R_getVarEx(Rf_install(ns.c_str()), R_NamespaceRegistry, FALSE, R_NilValue));
-#endif
-            if (env == R_NilValue) {
+            if (env == R_NilValue)
                 stop("there is no namespace called \"%s\"", ns);
-            }
+#endif
             get_function(name, env);
         }