Yeah... That's a first for me. There have been a few very early 1899's seen with a crack in the receiver at the back of the bolt - one reason they switched from a straight backed bolt to a curved back bolt in 1908 apparently. But I've never seen one cracked up there.

The bolt locks up at the rear against the receiver, so I can't see a small crack at that pin really being an immediate danger to anything. But I have to wonder what happened to it, or what manufacturing flaw occurred, that would allow that crack in the first place. I'd be having a gunsmith look at it for sure.