Fix minor backward incompatibility

The key refactor changed the way unencrypted private keys with compressed
public key are stored in the wallet. Apparently older versions relied on
this to verify the correctness of stored keys.

Note that earlier pre-release versions do risk creating wallets that can
not be opened by 0.8.3 and earlier.
Merge pull request #2825 from sipa/incompatkey

Fix minor backward incompatibility