The owner of the Back Bay apartment building destroyed by a nine-alarm fire that killed two firefighters last month is suing a welding company it says was responsible for the blaze. Law enforcement officials have said that the welding company — identified in the lawsuit as D&J Iron Works in Malden — failed to get a permit from the City of Boston for work on a railing next door to the building that burned down. Applying for a permit would have required the welders to have outside supervision to ensure the work was done safely.