Events: validate descriptor before FD_SET()

Events: validate descriptor before FD_SET()

The check in ngx_select_init_conf() can be insufficient because it
doesn't cover descriptors for local files, which are not accounted
for in cycle->connection_n.  For instance, to reach this condition
with new connections, no reusable connections must be available for
recycling in ngx_get_connection().

The fix adopts a similar code from the win32 select module.