mpi: Fix mpi_set_cond and mpi_swap_cond .

* mpi/mpiutil.c (_gcry_mpi_set_cond, _gcry_mpi_swap_cond): Don't use
the operator of !!, but assume SET/SWAP is 0 or 1.

If the code for !! would include a branch, it spoils the purpose of
mpi_set_cond/mpi_swap_cond at all. It's better to make sure the use
of this function to be called with 0 or 1 for SET/SWAP. Note that it
conforms when SET/SWAP is the result of conditional expression of
mpi_test_bit.

• Reported-by: Taylor R Campbell.