Check if EK cert from NV Index is an valid certificate

We have seen some TPMs where the EK certificate for RSA2048 are padded with ones up to the length of 1600 bytes.

This causes the agent to fail to register, as we don't expect that large certificates and the full NVR is also not a valid certificate.
To work around this, one option is to parse the data to check the actual length of the DER data structure.