Yes Martin Luther was promised safe passage to and from the Council. The Council was originally scheduled to be held in Mantua, Italy, and to begin May 23, 1537 it was postponed and moved to Bolognia due to the plague, there was a long recess and the Council was finally re-conviened at Trent. So while the original safe passage was granted for Mantua, it was extended to Luther, and other Protestants to attend and explain their theologies. Throughout history many have been promised safe passage to and from various Councils to defend their new doctrines.