mpi:ec: Fix for use of ec_mulm_lli in _gcry_mpi_ec_get_affine.

* mpi/ec.c (_gcry_mpi_ec_get_affine): Resize X and Y.  Add
GCRYECC_FLAG_LEAST_LEAK flag.

• Fixes-commit: aa089ec89badcd74817e5008c66036b1e28674f5
• Reported-by: Kr0emer <kroemerqaq@163.com>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>