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/vim9script.c b/src/vim9script.c
index 557892c..1d7eea0 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -942,7 +942,8 @@
 
 	    if (HASHITEM_EMPTY(hi))
 		// new variable name
-		hash_add(&si->sn_all_vars.dv_hashtab, newsav->sav_key);
+		hash_add(&si->sn_all_vars.dv_hashtab, newsav->sav_key,
+							       "add variable");
 	    else if (sav != NULL)
 		// existing name in a new block, append to the list
 		sav->sav_next = newsav;
@@ -1033,7 +1034,7 @@
 	    else
 	    {
 		if (sav_prev == NULL)
-		    hash_remove(all_ht, all_hi);
+		    hash_remove(all_ht, all_hi, "hide variable");
 		else
 		    sav_prev->sav_next = sav->sav_next;
 		sv->sv_name = NULL;