fix segfault

src/event/ngx_event.c

diff --git a/src/event/ngx_event.c b/src/event/ngx_event.c
index e827f23..563186e 100644
--- a/src/event/ngx_event.c
+++ b/src/event/ngx_event.c
@@ -584,7 +584,7 @@
     ccf = (ngx_core_conf_t *) ngx_get_conf(cycle->conf_ctx, ngx_core_module);
     ecf = ngx_event_get_conf(cycle->conf_ctx, ngx_event_core_module);
 
-    if (ccf->worker_processes > 1 && ecf->accept_mutex) {
+    if (ccf->master && ccf->worker_processes > 1 && ecf->accept_mutex) {
         ngx_use_accept_mutex = 1;
         ngx_accept_mutex_held = 0;
         ngx_accept_mutex_delay = ecf->accept_mutex_delay;