Use existing session context if new is actually NULL (#2946)

SSL_set_SSL_CTX() doesn't currently tolerate the ctx argument being NULL (it would just crash in some cases). I was pondering handling this by just explicitly error out if it was NULL. But I realised upstream OpenSSL has a different behaviour: falls back to existing session context from ssl.

This is unnecessary complexity, but in the name of interoperability I did that instead of flipping to an error state.

Author: torben-hansen

ssl/ssl_lib.cc

@@ -2856,7 +2856,8 @@ SSL_CTX *SSL_set_SSL_CTX(SSL *ssl, SSL_CTX *ctx) {
   if (!ssl->config) {
     return NULL;
   }
-  if (ssl->ctx.get() == ctx) {
+
+  if (!ctx || ssl->ctx.get() == ctx) {
     return ssl->ctx.get();
   }