Corrected nullpo_retv -> nullpo_ret since npc_unload is int.

git-svn-id: https://svn.code.sf.net/p/rathena/svn/trunk@5648 54d463be-8e91-2dee-dedb-b68131a5f0ec

src/map/npc.c

@@ -1532,7 +1532,7 @@ static int npc_unload_ev(DBKey key,void *data,va_list ap) {
 
 int npc_unload (struct npc_data *nd)
 {
-	nullpo_retv(nd);
+	nullpo_ret(nd);
 
 	npc_remove_map (nd);
 	map_deliddb(&nd->bl);