Add check for missing HTTP block

[willgittoes-dd]

If your nginx config doesn't have a http block (eg it's for a mail server), we get a null pointer crash when initialising the worker.

  auto main_conf = static_cast<opentracing_main_conf_t *>(
      ngx_http_cycle_get_module_main_conf(cycle, ngx_http_opentracing_module));
  if (!main_conf->tracer_library.data) {
    return NGX_OK;
  }

#define ngx_http_cycle_get_module_main_conf(cycle, module)                    \
    (cycle->conf_ctx[ngx_http_module.index] ?                                 \
        ((ngx_http_conf_ctx_t *) cycle->conf_ctx[ngx_http_module.index])      \
            ->main_conf[module.ctx_index]:                                    \
        NULL)