branches/zip: lock_rec_insert_check_and_lock(): Avoid casting away constness.

Use page_rec_get_next_const() instead. This silences a gcc 4.2.4 warning.
Reported by Sunny Bains.

lock/lock0lock.c

@@ -4926,7 +4926,7 @@ lock_rec_insert_check_and_lock(
 	}
 
 	trx = thr_get_trx(thr);
-	next_rec = page_rec_get_next((rec_t*) rec);
+	next_rec = page_rec_get_next_const(rec);
 	next_rec_heap_no = page_rec_get_heap_no(next_rec);
 
 	lock_mutex_enter_kernel();