SSL: workaround for i2d_SSL_SESSION() negative values

i2d_SSL_SESSION() may unexpectedly return a negative value on
memory allocation failure, even though the NULL buffer was passed,
notably in LibreSSL.

Reported by szzzy.

src/event/ngx_event_openssl.c

@@ -4462,7 +4462,7 @@ ngx_ssl_new_session(ngx_ssl_conn_t *ssl_conn, ngx_ssl_session_t *sess)
 
     /* do not cache too big session */
 
-    if (len > NGX_SSL_MAX_SESSION_SIZE) {
+    if (len <= 0 || len > NGX_SSL_MAX_SESSION_SIZE) {
         return 0;
     }
 

src/http/ngx_http_upstream_round_robin.c

@@ -1196,7 +1196,7 @@ ngx_http_upstream_save_round_robin_peer_session(ngx_peer_connection_t *pc,
 
         /* do not cache too big session */
 
-        if (len > NGX_SSL_MAX_SESSION_SIZE) {
+        if (len <= 0 || len > NGX_SSL_MAX_SESSION_SIZE) {
             return;
         }
 

src/stream/ngx_stream_upstream_round_robin.c

@@ -1027,7 +1027,7 @@ ngx_stream_upstream_save_round_robin_peer_session(ngx_peer_connection_t *pc,
 
         /* do not cache too big session */
 
-        if (len > NGX_SSL_MAX_SESSION_SIZE) {
+        if (len <= 0 || len > NGX_SSL_MAX_SESSION_SIZE) {
             return;
         }