I apologize to the mods in advance, but I believe this only requires a rather brief answer: you don't load a crossbow until after you've cocked it. It shouldn't even be possible to do so for the type of crossbow that OP is thinking of (which would be held against the ground while pulling the string back by putting a foot through a metal loop at the front); crossbow bolt nocks don't clip to the string in the way that some arrows (especially modern ones) do. They would fall off, even if the crossbowman ignored the most obvious safety procedure.

Repeating crossbows would be cocked while loaded, but the cocking motion is performed while the bolt is facing away from the shooter, so it would be nearly impossible to shoot oneself in the foot by accident with one of them either.