Safeguard against calling p255 reduction with single-width MPI
Safeguard against calling p255 reduction with single-width MPI

(In this case, there's nothing to do anyway since we only do a
quasi-reduction to N+1 limbs)

Signed-off-by: Hanno Becker <4d488caff78d6184f59bbe6e137e34d4606b9fe8@arm.com>

ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls