scd: Fix use of SCardListReaders for PC/SC.
* scd/apdu.c (open_pcsc_reader): Initialize NREADER.
Backport master commit of:
1b1684cf6192d9edb90a54ebe4a0e66b3d59a44b
- Reported-by: Ludovic Rousseau
- GnuPG-bug-id: T5979
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>