scd,pcsc: Fix error handling for a reader with reader-port.

* scd/apdu.c (apdu_open_reader): Make sure dl->idx is always
incremented to handle error from open_pcsc_reader correctly.

• Reported-by: Anže Jenšterle
• GnuPG-bug-id: T5758
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>