fx: namemap: fix nullptr deference in fx_namemap_first

fx/namemap.c

@@ -141,6 +141,10 @@ struct fx_namemap_entry *fx_namemap_first(const struct fx_namemap *map)
 		struct __fx_namemap_entry,
 		first,
 		e_node);
+	if (!entry) {
+		return NULL;
+	}
+
 	if (entry->e_flags == MAP_ENTRY_ITEM) {
 		return (struct fx_namemap_entry *)entry;
 	}