patch 9.0.0874: using freed memory when executing unmenu at more prompt

Problem:    Using freed memory when executing unmenu at the more prompt.
Solution:   Do not clear menus while listing them. (closes #11439)
diff --git a/src/errors.h b/src/errors.h
index 88db285..99247b6 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3335,3 +3335,7 @@
 #endif
 EXTERN char e_cannot_change_mappings_while_listing[]
 	INIT(= N_("E1309: Cannot change mappings while listing"));
+#if defined(FEAT_MENU)
+EXTERN char e_cannot_change_menus_while_listing[]
+	INIT(= N_("E1310: Cannot change menus while listing"));
+#endif