Reset $cmd_num when connection is closed

We want command ID counter to start over when a new connection is being made

Author: alecpl

program/lib/Roundcube/rcube_imap_generic.php

@@ -426,6 +426,7 @@ protected function closeSocket()
         if ($this->fp) {
             fclose($this->fp);
             $this->fp = null;
+            $this->cmd_num = 0;
         }
     }