Fix shutdown when ngx_pagespeed is completely disabled.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/1120

src/ngx_pagespeed.cc

@@ -3067,7 +3067,9 @@ void ps_exit_child_process(ngx_cycle_t* cycle) {
   ps_main_conf_t* cfg_m = static_cast<ps_main_conf_t*>(
       ngx_http_cycle_get_module_main_conf(cycle, ngx_pagespeed));
   NgxBaseFetch::Terminate();
-  cfg_m->driver_factory->ShutDown();
+  if (cfg_m->driver_factory != NULL) {
+    cfg_m->driver_factory->ShutDown();
+  }
 }
 
 // Called when nginx forks worker processes.  No threads should be started