Events: fixed copying of old events in poll init.

Previously, the code incorrectly assumed "ngx_event_t *" elements
instead of "struct pollfd".

This is mostly cosmetic change, as this code is never called now.
diff --git a/src/event/modules/ngx_poll_module.c b/src/event/modules/ngx_poll_module.c
index 4e03dab..b46ab53 100644
--- a/src/event/modules/ngx_poll_module.c
+++ b/src/event/modules/ngx_poll_module.c
@@ -84,7 +84,7 @@
         }
 
         if (event_list) {
-            ngx_memcpy(list, event_list, sizeof(ngx_event_t *) * nevents);
+            ngx_memcpy(list, event_list, sizeof(struct pollfd) * nevents);
             ngx_free(event_list);
         }