CAIRO — An Islamic State affiliate released a video Wednesday threatening to kill a Croatian hostage if Egyptian authorities do not release “Muslim women” held in prison within 48 hours, a day before the country plans to unveil a highly promoted new extension of the Suez Canal. The video, circulated on social media by Islamic State sympathizers, shows a man wearing a yellow jumpsuit kneeling in the desert before a knife-wielding masked man in military fatigues.