Fixed server crash when monster use blacksmith skills (fixes #930)

* Special thanks @reunite-ro

src/map/skill.c

@@ -6810,7 +6810,7 @@ int skill_castend_nodamage_id (struct block_list *src, struct block_list *bl, ui
 	case BS_OVERTHRUST:
 		if (sd == NULL || sd->status.party_id == 0 || (flag & 1)) {
 			int weapontype = skill_get_weapontype(skill_id);
-			if (!weapontype || pc_check_weapontype(dstsd, weapontype)) {
+			if (!weapontype || !dstsd || pc_check_weapontype(dstsd, weapontype)) {
 				clif_skill_nodamage(bl, bl, skill_id, skill_lv,
 					sc_start2(src, bl, type, 100, skill_lv, (src == bl) ? 1 : 0, skill_get_time(skill_id, skill_lv)));
 			}