Use safe macro practices even though they really aren't necessary.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@187424 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_musiconhold.c

@@ -186,8 +186,8 @@ static struct ao2_container *mohclasses;
 
 static int reload(void);
 
-#define mohclass_ref(class,string)   (ao2_t_ref((class), +1, string), class)
-#define mohclass_unref(class,string) (ao2_t_ref((class), -1, string), (struct mohclass *) NULL)
+#define mohclass_ref(class,string)   (ao2_t_ref((class), +1, (string)), class)
+#define mohclass_unref(class,string) (ao2_t_ref((class), -1, (string)), (struct mohclass *) NULL)
 
 static void moh_files_release(struct ast_channel *chan, void *data)
 {