patch 8.0.0345: islocked('d.changedtick') does not work

Problem:    islocked('d.changedtick') does not work.
Solution:   Make it work.
diff --git a/src/buffer.c b/src/buffer.c
index 328c2b3..b394d8e 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -884,7 +884,7 @@
 
     if (di != NULL)
     {
-	di->di_flags |= DI_FLAGS_LOCK | DI_FLAGS_FIX | DI_FLAGS_RO;
+	di->di_flags |= DI_FLAGS_FIX | DI_FLAGS_RO;
 	di->di_tv.v_type = VAR_NUMBER;
 	di->di_tv.v_lock = VAR_FIXED;
 	di->di_tv.vval.v_number = 0;