Fixed skill usage keeping timer being set when skill id is 0 (#5341)

src/map/skill.cpp

@@ -11954,7 +11954,7 @@ TIMER_FUNC(skill_castend_id){
 		else
 			skill_castend_damage_id(src,target,ud->skill_id,ud->skill_lv,tick,flag);
 
-		if( sd && sd->skill_keep_using.skill_id == ud->skill_id ){
+		if( sd && sd->skill_keep_using.skill_id > 0 && sd->skill_keep_using.skill_id == ud->skill_id ){
 			sd->skill_keep_using.tid = add_timer( sd->ud.canact_tick + 100, skill_keep_using, sd->bl.id, 0 );
 		}