g10: Fix ECDH, clarifying the format.

* g10/ecdh.c (pk_ecdh_encrypt_with_shared_point): Returns error when
it's short.  Clarify the format.  Handle other prefixes correctly.

With the scdaemon's change, there is no case NBYTES < SECRET_X_SIZE.
This fixes the break of ECDH with X25519.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>