This is a simple economics question and turns on what building it up will cost you. For the moment, assume it is a crack (I'm not convinced it is). So the frame may last another year or two or it might last two weeks. If building yourself I'd go ahead, because you have nothing to lose, but if paying for a build, you need to decide how much life you need to get value from the outlay. Unfortunately there's no answer anyone here can give you, better than my 2 weeks or two years.

As for safety, I agree with others that the main hazard is inconvenience, especially if you live in a hilly area. If it cracks completely, it will still be safe to ride, but will only tolerate very light pedal pressure. If you're creative you can make emergency repairs to get you home, so even the inconvenience can be mitigated.

As for know whether it'a crack or not, the only way is to sand the area to bright bare steel, and use a contrast die to bring up the crack, and make a judgment call. If you have a friend who does "magneflux" he can give you a definitive answer over a few beers, but a commercial price might be more than the whole deal is worth.