Fix Perm16::random

Author: james-d-mitchell

include/hpcombi/perm16_impl.hpp

@@ -162,7 +162,7 @@ inline Perm16 Perm16::random(uint64_t n) {
     static std::mt19937 g(rd());
 
     Perm16 res = one();
-    auto ar = res.as_array();
+    auto &ar = res.as_array();
 
     std::shuffle(ar.begin(), ar.begin() + n, g);
     return res;