add back xfree call that was dropped as part of cleanup

Author: bvogelzang

ext/tiny_tds/client.c

@@ -196,6 +196,7 @@ static void rb_tinytds_client_free(void *ptr) {
     cwrap->closed = 1;
     cwrap->userdata->closed = 1;
   }
+  xfree(cwrap->userdata);
   xfree(ptr);
 }