While I very much doubt that it would lead to a child purposely self-inflicting an injury to get a treat, it's still a bad idea. For injuries that are not life threatening - try laughing at it. Laugh out loud. LOL. Obviously you can still show concern and care but if it's not life threatening, then what's the harm in teaching a skill to laugh at our clumsiness or unluckiness.?
Our kids sometimes laugh and cry at the same time. And now they laugh at me when I get hurt. Even when I want to cry. It's better than swearing and seems to take the edge off of the pain. At a young age it can be a cognitive dissonance distractor. At an older age it severely limits the "woe is me" factor.
My wife just crashed during a bike race and busted 3 ribs. That was the only time laughing was hard. And it made it even funnier because all she could say was "Don't make me laugh! It hurts to laugh!" Which only made us laugh more.