RSA_private_{en,de}crypt() can fail and will return -1 in that case.

RSA_private_{en,de}crypt() can fail and will return -1 in that case.
Check for this in the ca process and return a valid answer to the
relay process. This fixes rsae_send_imsg poll timeouts blocking relay
processes as seen by Mischa Peters and myself.
OK benno@