control/controlclient: return correct certificate

When searching for the matching client identity, the returned
certificate chain was accidentally set to that of the last identity
returned by the certificate store instead of the one corresponding to
the selected identity.

Also, add some extra error checking for invalid certificate chains, just
in case.

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f