scd: Make sure releasing PC/SC context.
* scd/apdu.c (release_pcsc_context): New. (close_pcsc_reader): Use release_pcsc_context. Add assertion. (apdu_dev_list_start): Replace call of close_pcsc_reader into release_pcsc_context, add condition.
- GnuPG-bug-id: T5416
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>