call SecTrustEvaluate before calling SecTrustGetCertificateCount

SecTrustGetCertificateCount's documentation says SecTrustEvaluate must be called before calling it. Without calling it, SecTrustGetCertificateCount returns 0 in OS X 10.7.
Merge pull request #1993 from minorblend/af_1.x

call SecTrustEvaluate before calling SecTrustGetCertificateCount