SecureTransport: handle NULL trust on success

The `SSLCopyPeerTrust` call can succeed but fail to return a trust
object if it can't load the certificate chain and thus cannot check the
validity of a certificate. This can lead to us calling `CFRelease` on a
`NULL` trust object, causing a crash.

Handle this by returning ECERTIFICATE.

src/stransport_stream.c

@@ -67,6 +67,9 @@ int stransport_connect(git_stream *stream)
 	if ((ret = SSLCopyPeerTrust(st->ctx, &trust)) != noErr)
 		goto on_error;
 
+	if (!trust)
+		return GIT_ECERTIFICATE;
+
 	if ((ret = SecTrustEvaluate(trust, &sec_res)) != noErr)
 		goto on_error;