PackageCollectionsSigning: avoid peeking into X509_STORE_CTX (#3730)

This uses the `CCryptoBoringSSL_X509_STORE_CTX_get_current_cert`
accessor to access the `current_cert` member.  This is purely a cleanup
change that will become important when swift-crypto is bumped as
BoringSSL has made more of the interfaces opaque.

Author: compnerd

Sources/PackageCollectionsSigning/Certificate/CertificatePolicy.swift

@@ -182,7 +182,7 @@ extension CertificatePolicy {
                 // Certs could have unknown critical extensions and cause them to be rejected.
                 // Check if they are tolerable.
                 if errorCode == X509_V_ERR_UNHANDLED_CRITICAL_EXTENSION {
-                    guard let cert = ctx?.pointee.current_cert else {
+                    guard let ctx = ctx, let cert = CCryptoBoringSSL_X509_STORE_CTX_get_current_cert(ctx) else {
                         return result
                     }