patch 9.0.0949: crash when unletting a variable while listing variables

Problem:    Crash when unletting a variable while listing variables.
Solution:   Disallow changing a hashtable while going over the entries.
            (closes #11435)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index cb52438..217147a 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2366,7 +2366,7 @@
 						  NULL, FALSE))
 		    status = FAIL;
 		else
-		    dictitem_remove(d, di);
+		    dictitem_remove(d, di, "unlet");
 	    }
 	}
     }