ISTANBUL — Two explosions struck Saturday night outside a major soccer stadium in Istanbul after fans had gone home, an attack that wounded at least 20 police officers, Turkish authorities said. One of the blasts was thought to be a car bomb and the second appeared to have been caused by a suicide bomber. Police cordoned off the area as smoke rose from behind the newly built Vodafone Arena Stadium, known colloquially as Besiktas Stadium after the local team and neighborhood.