Skip set.seed() if NA_integer_ is given.

This should address mlampros#52 but currently it does not work because
`NA_integer_` is implemented as the maximum negative value, and
a negative seed causes an error "negative length vectors are not allowed".
`base::set.seed()` accepts negative seeds without any problem,
which indicates ClusterR is doing something wrong with negative seeds.

inst/include/ClusterRHeader.h

@@ -117,6 +117,7 @@ namespace clustR {
 
       void set_seed(int seed) {
 
+        if (Rcpp::all(Rcpp::is_na(Rcpp::IntegerVector(seed)))) return;
         Rcpp::Environment base_env("package:base");
         Rcpp::Function set_seed_r = base_env["set.seed"];
         set_seed_r(seed);

inst/include/affinity_propagation.h

@@ -58,6 +58,7 @@ class Affinity_Propagation {
 //---------------------------------------------------------------------------------
 
 void Affinity_Propagation::set_seed(int seed) {
+  if (Rcpp::all(Rcpp::is_na(Rcpp::IntegerVector(seed)))) return;
   Rcpp::Environment base_env("package:base");
   Rcpp::Function set_seed_r = base_env["set.seed"];
   set_seed_r(seed);