Use SecTrust return status code at runtime

The `SecTrust` functions in use within AFSecurityPolicy.h only make use of the returned status code when `NS_BLOCK_ASSERTIONS` is not defined. The use of `NS_BLOCK_ASSERTIONS` to silence static analyzer warnings about unused values is convenient, however it is also suppressing the larger issues which are that `AFPublicKeyForCertificate`, `AFPublicKeyTrustChainForServerTrust` and `AFServerTrustIsValid` fail to detect a runtime status code that is not `errSecSuccess` and to return early.