Oh its not impossible, I mean fire is why all those marshalls hang around with extinguishers and *occasionally* you see them actually use them on real flames.
It is however a question of risk vs probability that we all make.
If you were to look at the hazards which actually impact trackside photography based on frequency and therefore probability I'd say it probably ranks something like this:
1) Sunburn
2) Broken/twisted ankles
3) Gravel in the eye
4) Vehicle Debris hitting you
5) Tyrewall hitting you
6) Vehicle hitting you
7) Fireball hitting you
So, sun block, boots, ballistic glasses, helmet and body armour probably all should be purchased before flame proof overalls.
Unlike the marshalls, we aren't actually required to go leaping into burning cars...