gpg: Fix gpg_mpi_write for the unused opaque case.

* g10/build-packet.c (gpg_mpi_write): Take care of the fact that
get_opaque already returns a bit-exact value.

• Fixes-commit: ab17f7b6c392782718f57eaea94fc18a0ff49389
• Reported-by: Falko Strenzke <falko.strenzke@mtg.de>