I guess that it occurs when favicon/thumbnail for some feed is resized and libpng does not support it. I would still say it is either a bug in libpng or PHP.

Try adding the following code before $f3->run(); in index.php:

https://gist.github.com/jtojnar/cbf19757c01b7aaa47832d5baf26f3e3

I added the lines in the index.php.
Now I only get the debug messages as result.
What part of the debug messages do you need to investigate further?