InnoDB: Enable ut_ad() assertions in MySQL debug builds.

innobase/include/univ.i:
  InnoDB: Define UNIV_DEBUG when DBUG_ON (in MySQL) is defined.
  This enables InnoDB debug assertions in debug builds of mysqld.

innobase/include/univ.i

@@ -86,8 +86,10 @@ memory is read outside the allocated blocks. */
 
 /* Make a non-inline debug version */
 
+#ifdef DBUG_ON
+# define UNIV_DEBUG
+#endif /* DBUG_ON */
 /*
-#define UNIV_DEBUG
 #define UNIV_SYNC_DEBUG
 #define UNIV_MEM_DEBUG