Suspicious about sign_ecdsa

[NIIBE Yutaka]

Very minor thing.

While I read the function (sign_ecdsa of ecc.c), I think that the
error code of following part is not appropriate.

          if (_gcry_mpi_ec_get_affine (x, NULL, &I, ctx))
            {
              if (DBG_CIPHER)
                log_debug ("ecc sign: Failed to get affine coordinates\n");
              err = GPG_ERR_BAD_SIGNATURE;
              goto leave;
            }

This error occurs only if we have coding error or the curve is
invalid (... or hardware error).

GPG_ERR_INTERNAL would be appropriate.
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