Fix #29
Fix #29

Only Units with UNIT_FLAG_SKINNABLE flag can be skinned. Fix a TO_CREATURE() cast that could be called with a Player as argument.

lbfamous/arcemu,lbfamous/arcemu,sysuzhang/arcemu,arcemu/arcemu,arcemu-ng/arcemu,dberga/arcbliz,arcemu/arcemu,arcemu-ng/arcemu,JoMiro/arcemu,JoMiro/arcemu,arcemu-ng/arcemu,JoMiro/arcemu,arcemu/arcemu,lbfamous/arcemu,lbfamous/arcemu,sysuzhang/arcemu,JoMiro/arcemu,arcemu/arcemu,lbfamous/arcemu,JoMiro/arcemu,arcemu-ng/arcemu,sysuzhang/arcemu,sysuzhang/arcemu,dberga/arcbliz,JoMiro/arcemu,arcemu/arcemu,dberga/arcbliz,arcemu/arcemu,arcemu-ng/arcemu,arcemu-ng/arcemu,dberga/arcbliz,lbfamous/arcemu,sysuzhang/arcemu,dberga/arcbliz,sysuzhang/arcemu