Perform the null check right after the memory allocation

Closes #876

Author: Anchels

opentracing/src/ngx_http_opentracing_module.cpp

@@ -253,8 +253,8 @@ static void *create_opentracing_main_conf(ngx_conf_t *conf) noexcept {
   auto main_conf = static_cast<opentracing_main_conf_t *>(
       ngx_pcalloc(conf->pool, sizeof(opentracing_main_conf_t)));
   // Default initialize members.
-  *main_conf = opentracing_main_conf_t();
   if (!main_conf) return nullptr;
+  *main_conf = opentracing_main_conf_t();
   return main_conf;
 }