mbedtls_mpi::s field

Sign: -1 if the mpi is negative, 1 otherwise. The number 0 must be represented with `s = +1`. Although many library functions treat all-limbs-zero as equivalent to a valid representation of 0 regardless of the sign bit, there are exceptions, so bignum functions and external callers must always set \c s to +1 for the number zero. Note that this implies that calloc() or `... = {0}` does not create a valid MPI representation. You must call mbedtls_mpi_init().