> has to be changed so a person isn't liable if he deletes or reports the material as soon as he first becomes aware of it

AFAIK that's more or less how it works today as a practical matter. The law recognizes this situation as an affirmative defense, but does not make it impossible to be charged.

To be an affirmative defense it has to be reported immediately or destroyed, constitute three images or less, and not be sent to anyone [other than law enforcement]. Interestingly, NCMEC doesn't necessarily count for that -- if you find yourself in this situation and you want to report something, call your local police.