William Saliba has now sent Arsenal a warning about FC Porto ahead of the Gunners hosting the Portuguese team in the second leg of their Champions League last 16 tie. The Dragoes visit Emirates Stadium on Tuesday night and will travel with a 1-0 advantage on aggregate. Porto secured a slender advantage from the reverse tie on February 21 as Galeno curled in a precise, curling shot. He finessed past goalkeeper David Raya in the 93rd minute. Porto scored in the final seconds after Gabriel Martinelli cheaply turned over the possession as Arsenal tried to counter. Centre-half Otavio easily interce…