ecc: Check the input length for the point.

* cipher/ecc-misc.c (_gcry_ecc_mont_decodepoint): Check the length
of valid point representation.

In the use case of GnuPG, ECDH decryption for anonymous recipient may
try to decrypt with different curves. When the input data of
ephemeral key does not match one of the private key, it should return
GPG_ERR_INV_OBJ.

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