Fix typo in ssl_cert_cache_chain_certs.

After e325c3f4712b23de688d2eea7d7549b65a610605, this typo bites and
causes SSL_CTX_get_extra_chain_certs to return an empty stack.

Change-Id: I6aa7093d1ca4f3ba0f520a644b14de5b3a3ccaa6
Reviewed-on: https://boringssl-review.googlesource.com/27604
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_x509.cc

@@ -900,7 +900,7 @@ static int ssl_cert_cache_chain_certs(CERT *cert) {
   assert(cert->x509_method);
 
   if (cert->x509_chain != nullptr ||
-      cert->chain != nullptr ||
+      cert->chain == nullptr ||
       sk_CRYPTO_BUFFER_num(cert->chain.get()) < 2) {
     return 1;
   }