Fix a ssl session leak due to OOM in lh_SSL_SESSION_insert

- s == NULL can mean c is a new session *or* lh_insert was
  unable to create a hash entry.
- use lh_SSL_SESSION_retrieve to check for this error condition.
- If it happens simply remove the extra reference again.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2138)

ssl/ssl_sess.c

@@ -678,6 +678,15 @@ int SSL_CTX_add_session(SSL_CTX *ctx, SSL_SESSION *c)
          * obtain the same session from an external cache)
          */
         s = NULL;
+    } else if (s == NULL &&
+               lh_SSL_SESSION_retrieve(ctx->sessions, c) == NULL) {
+        /* s == NULL can also mean OOM error in lh_SSL_SESSION_insert ... */
+
+        /*
+         * ... so take back the extra reference and also don't add
+         * the session to the SSL_SESSION_list at this time
+         */
+        s = c;
     }
 
     /* Put at the head of the queue unless it is already in the cache */