By Valentina Za MILAN (Reuters) -UniCredit cheered investors by saying it would soon start a 1.6 billion euro ($1.7 billion) share buyback put at risk by the Ukraine war, even as it set aside almost as much to cover the cost of quitting Russia. One of Europe’s banks most exposed to Russia, UniCredit said the 1.3-billion-euro provision booked to first quarter results covered more than 70% of the capital hit it could suffer in a worst-case scenario. It could now write off in full its Russian unit, were it to be nationalised or sold for free, with no further impact on capital. “We now have the fl…