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.
diff --git a/src/stransport_stream.c b/src/stransport_stream.c
index 66be58a..008bdfb 100644
--- a/src/stransport_stream.c
+++ b/src/stransport_stream.c
@@ -67,6 +67,9 @@
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;
