branches/zip: fil_close(): Add #ifndef UNIV_HOTBACKUP around a debug

assertion on mutex.magic_n.  InnoDB Hot Backup is a single-threaded
program and does not contain mutexes.  This change allows InnoDB Hot
Backup to be compiled with UNIV_DEBUG.

Suggested by Michael Izioumtchenko.

fil/fil0fil.c

@@ -4786,8 +4786,10 @@ void
 fil_close(void)
 /*===========*/
 {
+#ifndef UNIV_HOTBACKUP
 	/* The mutex should already have been freed. */
 	ut_ad(fil_system->mutex.magic_n == 0);
+#endif /* !UNIV_HOTBACKUP */
 
 	hash_table_free(fil_system->spaces);