Fix meme generation for palette PNGs

Summary: Fixes T4125. Indexed / palette PNGs may fail to allocate a proper black for drawing an image border.

Test Plan: {F83066}

Reviewers: btrahan, asukhachev

Reviewed By: asukhachev

CC: aran

Maniphest Tasks: T4125

Differential Revision: https://secure.phabricator.com/D7604