sm: Support decryption of ECDH data using a smartcard.
* sm/decrypt.c (ecdh_decrypt): Add arg nbits and detect bare secret. (prepare_decryption): Add arg nbits and pass on. (gpgsm_decrypt): Pass size of curve to prepare_decryption.
- GnuPG-bug-id: T4098
- Signed-off-by: Werner Koch <wk@gnupg.org>