mpi:ec: Keep A untouched in ec_get_a_is_pminus3.

* mpi/ec.c (ec_get_a_is_pminus3): Change the calculation
so that it keeps the MPI of A untouched.

Note that mpi_cmp with A may normalize the MPI.

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