I did a little googling, and destroying the porn would actually be the right thing to do and so shouldn't get you in trouble. According to "18 U.S. Code § 2252A - Certain activities relating to material constituting or containing child pornography", an affirmative defense is not having much and either deleting it promptly or reporting it:

  (d) Affirmative Defense.— It shall be an affirmative defense to a charge of
      violating subsection (a)(5) that the defendant—
    (1) possessed less than three images of child pornography;
    and
    (2) promptly and in good faith, and without retaining or allowing any person, 
       other than a law enforcement agency, to access any image or copy thereof—
        (A) took reasonable steps to destroy each such image;
        or
        (B) reported the matter to a law enforcement agency
            and afforded that agency access to each such image.

(source: https://www.law.cornell.edu/uscode/text/18/2252A)