Fixed possible null pointer dereference in Taekwon Mission skill logic. Fixes #3996

Thanks to @cydh

src/map/mob.cpp

@@ -2956,9 +2956,9 @@ int mob_dead(struct mob_data *md, struct block_list *src, int type)
 		if (sd) {
 			struct mob_db *mission_mdb = mob_db(sd->mission_mobid);
 
-			if ((sd->mission_mobid == md->mob_id) ||
-				(battle_config.taekwon_mission_mobname == 1 && mission_mdb && status_get_race2(&md->bl) == RC2_GOBLIN && mission_mdb->race2 == RC2_GOBLIN) ||
-				(battle_config.taekwon_mission_mobname == 2 && mob_is_samename(md, sd->mission_mobid)))
+			if ((sd->mission_mobid == md->mob_id) || (mission_mdb != nullptr &&
+				((battle_config.taekwon_mission_mobname == 1 && status_get_race2(&md->bl) == RC2_GOBLIN && mission_mdb->race2 == RC2_GOBLIN) ||
+				(battle_config.taekwon_mission_mobname == 2 && strcmp(mob_db(md->mob_id)->jname, mission_mdb->jname) == 0))))
 			{ //TK_MISSION [Skotlex]
 				if (++(sd->mission_count) >= 100 && (temp = mob_get_random_id(MOBG_Branch_Of_Dead_Tree, static_cast<e_random_monster_flags>(RMF_CHECK_MOB_LV|RMF_MOB_NOT_BOSS|RMF_MOB_NOT_SPAWN), sd->status.base_level)))
 				{

src/map/mob.hpp

@@ -338,7 +338,6 @@ void mob_heal(struct mob_data *md,unsigned int heal);
 
 #define mob_stop_walking(md, type) unit_stop_walking(&(md)->bl, type)
 #define mob_stop_attack(md) unit_stop_attack(&(md)->bl)
-#define mob_is_samename(md, mid) (strcmp(mob_db((md)->mob_id)->jname, mob_db(mid)->jname) == 0)
 
 void mob_clear_spawninfo();
 void do_init_mob(void);