ecc: Detect the use of a Montgomery curve earlier in ecc_verify.

* cipher/ecc.c (ecc_verify): Do not allow a Montgomery curve.

This is required so that we don't run into the log_fatal in
_gcry_mpi_ec_get_affine. There is no problem in Libgcrypt 1.8 because
there this error case is already detected due to a slightly different
code.

• Signed-off-by: Werner Koch <wk@gnupg.org>