A Frontier Airlines plane was evacuated after staff found a gun magazine aboard in Denver — days after a flight struck and killed a trespasser on the tarmac at the same airport.Passengers deplaned from a Frontier flight from Denver to Phoenix on Sunday after staff made the startling discovery, a company spokesperson confirmed.Passengers were screened “as a matter of precaution,” and no actual firearms were found aboard.“Our preliminary investigation indicates that the magazine belongs to a law enforcement officer,” Frontier