[fix] Throw an error when trying to access the clients of a dynamic namespace (#3355)

Accessing the clients of a dynamic namespace throws because doing `io.of(/your-regex/g)` returns a namespace with no adapter and the clients methods tries to access `namespace.adapter.clients`.

Author: 0xClpz

lib/namespace.js

@@ -262,6 +262,9 @@ Namespace.prototype.write = function(){
  */
 
 Namespace.prototype.clients = function(fn){
+  if(!this.adapter){
+    throw new Error('No adapter for this namespace, are you trying to get the list of clients of a dynamic namespace?')
+  }
   this.adapter.clients(this.rooms, fn);
   // reset rooms for scenario:
   // .in('room').clients() (GH-1978)