Bug: credentials are stored when key decoding failed resulting in continuous fails on main page

[Christian Kahlo]

When trying to register a credential containing a public key with its parameters encoded as strings instead of integers the credential is stored with an empty key.
When refreshing the page the URL https://webauthndemo.appspot.com/RegisteredKeys is called to display already registered credentials.
The code throws NullPointerExceptions when trying to encode the public key x and y coordinates with EccKey getX(), getY() or encode(). This renders the demo application unusable.

[cpiper]

Thanks for the report. In b16354e I am now rejecting such invalid entries. Invalid entries should be cleared from the database, but let me know if you still run into issues.