bugfix: fixed a ctx nullity test caught by cppcheck and reported by Ilya Shipitsin in #17.

Author: agentzh

src/ngx_stream_lua_util.c

@@ -561,17 +561,17 @@ ngx_stream_lua_session_cleanup(ngx_stream_lua_ctx_t *ctx, int forcible)
     ngx_stream_session_t            *s;
     ngx_stream_lua_main_conf_t      *lmcf;
 
-    s = ctx->session;
-
-    ngx_log_debug1(NGX_LOG_DEBUG_STREAM, s->connection->log, 0,
-                   "stream lua session cleanup: forcible=%d", forcible);
-
     /*  force coroutine handling the session quit */
     if (ctx == NULL) {
         dd("ctx is NULL");
         return;
     }
 
+    s = ctx->session;
+
+    ngx_log_debug1(NGX_LOG_DEBUG_STREAM, s->connection->log, 0,
+                   "stream lua session cleanup: forcible=%d", forcible);
+
     lmcf = ngx_stream_get_module_main_conf(s, ngx_stream_lua_module);
 
 #if 1