Allow shutdown to unload modules that register bucket scheme's or codec's.

* Change __ast_module_shutdown_ref to be NULL safe (11+).
* Allow modules that call ast_bucket_scheme_register or ast_codec_register
  to be unloaded during graceful shutdown only (13+ only).

ASTERISK-24796 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4428/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@432058 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/loader.c

@@ -1378,10 +1378,12 @@ struct ast_module *__ast_module_ref(struct ast_module *mod, const char *file, in
 
 void __ast_module_shutdown_ref(struct ast_module *mod, const char *file, int line, const char *func)
 {
-	if (!mod->flags.keepuntilshutdown) {
-		__ast_module_ref(mod, file, line, func);
-		mod->flags.keepuntilshutdown = 1;
+	if (!mod || mod->flags.keepuntilshutdown) {
+		return;
 	}
+
+	__ast_module_ref(mod, file, line, func);
+	mod->flags.keepuntilshutdown = 1;
 }
 
 void __ast_module_unref(struct ast_module *mod, const char *file, int line, const char *func)