random: materialise the offset temporary for container choose

This helps a little in some debugging situations and isn't terrifically
expensive.

random.hpp

@@ -165,9 +165,11 @@ namespace cruft::random {
         if (data.empty ())
             return data.end ();
 
-        return std::next (
-            data.begin (),
-            uniform (typename ContainerT::size_type {0}, data.size () - 1)
+        auto const offset = uniform (
+            typename ContainerT::size_type {0},
+            data.size () - 1
         );
+
+        return std::next (data.begin (), offset);
     }
-};
+}