Fix #773 New sessions' operations hanging on iOS 8

On the iOS8, following code does not return any certificates on second (and further) IMAP session (this is strange, as the first session works well):

<code>
CFReadStreamCopyProperty(cfstream_data->readStream, kCFStreamPropertySSLPeerCertificates);
</code>

As kCFStreamPropertySSLPeerCertificates seems to be deprecated since iOS4, the alternative method 
to retrieve certificates from stream can be used and seems to work well both on iOS8 and iOS7:

<code>
CFReadStreamCopyProperty(cfstream_data->readStream, kCFStreamPropertySSLPeerTrust);
SecTrustGetCertificateCount(secTrust);
SecTrustGetCertificateAtIndex(secTrust, i);
</code>

BR,
Maksim
What happens if NULL is returned?