mpi: More fix of off-by-one mistake mpi_invm_pow2.

* mpi/mpi-inv.c (mpi_invm_pow2): Avoid out-of-band read/write.

• Suggested-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
• Reported-by: Guido Vranken <guidovranken@gmail.com>
• Fixes-commit: 3bb9f74764b3626ed1116fc7e517921232d6be54
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>