Fix a crash when attempting to sign a raw transaction with a zero privkey

These commands were crashing clamd:

    clamd signrawtransaction "$raw" '[]' '["5TY1TqhFLax7w7Hoa6YtrejzoAuWTf5Tf1RKSvVED9tadmUMYjX"]'
    clamd signrawtransaction "$raw" '[]' '["Lg7s3TveHSrHbpmAZ5Vmt9an6nim2f4gekrLRR5bJeEfT2fXkRoT"]'

That's 0 encoded as a privkey (compressed and not). It's not a valid key, and so should be rejected.