mpi: Use secure MPI in _gcry_mpi_assign_limb_space.
* mpi/mpiutil.c (_gcry_mpi_assign_limb_space): Use secure memory when destination is secure memory.
Backport master commit of:
6e77b09cff56dd357cd2be332e3a2af200190382
- Reported-by: Pavel Kohout, Aisle Research, www.aisle.com
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>