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/if_ruby.c b/src/if_ruby.c
index feb1d50..51cfff1 100644
--- a/src/if_ruby.c
+++ b/src/if_ruby.c
@@ -1799,7 +1799,7 @@
     if (di == NULL || ruby_convert_to_vim_value(val, &di->di_tv) != OK
 						     || dict_add(d, di) != OK)
     {
-	d->dv_hashtab.ht_error = TRUE;
+	d->dv_hashtab.ht_flags |= HTFLAGS_ERROR;
 	return ST_STOP;
     }
     return ST_CONTINUE;
@@ -1879,7 +1879,7 @@
 		    return FAIL;
 
 		rb_hash_foreach(val, convert_hash2dict, (VALUE)d);
-		if (d->dv_hashtab.ht_error)
+		if (d->dv_hashtab.ht_flags & HTFLAGS_ERROR)
 		{
 		    dict_unref(d);
 		    return FAIL;