am a0bb96c3: Update libpng to 1.2.38 to patch a minor security issue.

Merge commit 'a0bb96c34e65378853ee518bac502842d26c2d1a'

* commit 'a0bb96c34e65378853ee518bac502842d26c2d1a':
  Update libpng to 1.2.38 to patch a minor security issue.