Add missing check to multipart decrypt
Add missing check to multipart decrypt

Ensure that the test actually does something, rather than skipping both
parts, also add comment to this effect.

Signed-off-by: Paul Elliott <c95151bd40fac1aa138ed1651e2873f91d2e1899@arm.com>

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