Fix map-crash when floating npc use *npcspeed()

- script command *npcspeed() are not applicable for floating npc (without
coordinate/location).

Author: Emistry

src/map/script.c

@@ -15910,20 +15910,22 @@ static BUILDIN(npctalk)
 // change npc walkspeed [Valaris]
 static BUILDIN(npcspeed)
 {
-	struct npc_data* nd;
-	int speed;
-
-	speed = script_getnum(st,2);
-	nd = map->id2nd(st->oid);
+	struct npc_data *nd = map->id2nd(st->oid);
+	int speed = script_getnum(st, 2);
 
 	if (nd != NULL) {
 		unit->bl2ud2(&nd->bl); // ensure nd->ud is safe to edit
+		if (nd->ud == NULL) {
+			ShowWarning("buildin_npcspeed: floating NPC don't have unit data.\n");
+			return false;
+		}
 		nd->speed = speed;
 		nd->ud->state.speed_changed = 1;
 	}
 
 	return true;
 }
+
 // make an npc walk to a position [Valaris]
 static BUILDIN(npcwalkto)
 {