fix: Pad secret keys with zeros before sending them to NetHSM
This fixes a rare bug when importing OpenPGP keys with a scalar part too short. Since OpenPGP does not preserve leading zeros and NetHSM expects leading zeros this caused errors on import when the generated scalar started with a zero value.
Pad the scalars to appropriate size before processing them further. Additionally check for a scalar values that are not compatible with the target curve.
Add a test case that was captured during a flaky test run.
Fixes: #70 (closed)