mpi: Fix mpi_pow alternative implementation.

* mpi/mpi-pow.c
  [USE_ALGORITHM_SIMPLE_EXPONENTIATION] (_gcry_mpi_powm): Use
  mpi_set_cond.

Limbs of RES may be allocated more before the call of mpi_pow,
but it only uses the space of SIZE.

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