Assert presence of server certificate in Certificate writer
Assert presence of server certificate in Certificate writer

The server-side `Certificate` handshake message writer checks
whether a certificate is present, and if not fails with:

```
   MBEDTLS_ERR_SSL_CERTIFICATE_REQUIRED
```

This should never happen, since the library checks the presence
of a suitable certificate before picking a ciphersuite. It is
therefore more suitable to convert this check into an assertion,
and fail with MBEDTLS_ERR_SSL_INTERNAL_ERROR upon failure.

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

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