Invalid PCR selection during key.Attest()

[daniel-weisse]

Assuming we are running on a system with a TPM that does not support the full range of PCR Selections:

$ sudo tpm2_getcap pcrs
selected-pcrs:
  - sha1: [ ]
  - sha256: [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 ]
  - sha384: [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 ]
  - sha512: [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 ]

Then calling key.Attest(AttestOpts{Nonce:nonce}) fails with the following error:

failed to verify quote: given PCRs and Quote do not have the same PCR selection

The error is fixed by adjusting implementedPCRs() to only add a PCRSelection to the implemented PCRs list if a given PCRSelection is not an empty list.

I can open a PR to fix this.