Avoid seemingly-possible overflow
Avoid seemingly-possible overflow

By looking just at that test, it looks like 2 + dn_size could overflow. In
fact that can't happen as that would mean we've read a CA cert of size is too
big to be represented by a size_t.

However, it's best for code to be more obviously free of overflow without
having to reason about the bigger picture.

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