(stockholder in residential cooperative housing corporation could claim casualty loss deduction related to $26,390 assessment paid so that retaining wall in common area could be repaired; stockholder had sufficient property interest in wall under state (NY) law; case remanded so Tax Court could determine if loss was actually caused by casualty; petitioner leased apartment building from housing corporation that she owned stock in and court determined that stockholder's rights were granted by lease agreement which gave her right to use grounds in complex and exclude persons that are not tenants or their guests).