Fix a segmentation fault if the private key file is corrupted

When the private key file is corrupted, cf-promises and cf-agent generate a segmentation fault:
Without the patch:

    $ cf-promises 
    2014-01-29T07:29:57+0100    error: Error reading private key. (PEM_read_RSAPrivateKey: too long)
    Segmentation fault

With the patch:

    $ cf-promises 
    2014-01-29T07:25:22+0100    error: Error reading private key. (PEM_read_RSAPrivateKey: too long)
    2014-01-29T07:25:22+0100    error: Fatal CFEngine error: Couldn't read private key from file /home/loic/.cfagent/ppkeys/localhost.priv

Feedback is welcome :-)