Crashfix for negative skill id values (bugreport:2984)

git-svn-id: https://svn.code.sf.net/p/rathena/svn/trunk@13684 54d463be-8e91-2dee-dedb-b68131a5f0ec

src/map/skill.c

@@ -9782,7 +9782,7 @@ struct skill_unit_group* skill_initunitgroup (struct block_list* src, int count,
 	struct skill_unit_group* group;
 	int i;
 
-	if(skilllv <= 0) return 0;
+	if(skillid <= 0 || skilllv <= 0) return 0;
 
 	nullpo_retr(NULL, src);
 	nullpo_retr(NULL, ud);