Don't let a NULL mean the initial SSL_CTX in SSL_set_SSL_CTX.

We broke this a while back and nothing noticed. (Note dereference just
above.) Therefore I guess we don't need to support it.

Change-Id: I501d43825e89acb5f7f13998541dc8ff59452a99
Reviewed-on: https://boringssl-review.googlesource.com/31144
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>
diff --git a/ssl/ssl_lib.cc b/ssl/ssl_lib.cc
index a126561..120c276 100644
--- a/ssl/ssl_lib.cc
+++ b/ssl/ssl_lib.cc
@@ -2260,10 +2260,6 @@
     return NULL;
   }
 
-  if (ctx == NULL) {
-    ctx = ssl->session_ctx.get();
-  }
-
   UniquePtr<CERT> new_cert = ssl_cert_dup(ctx->cert.get());
   if (!new_cert) {
     return nullptr;